diff --git a/CHANGELOG.md b/CHANGELOG.md
index 427daec2..6344ab10 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,13 +1,27 @@
 # Change Log
 
+## Unreleased
+
+### Added
+
+- Added `Duration.to_iso8601_string()` method to output a ISO 8601 string representation of a duration.
+
+### Fixed
+
+- Fixed Duration.microseconds not returning an int in python2
+
+
 ## [2.0.4] - 2018-10-30
 
+### Added
+
+- Added support for parsing padded 2-digit days of the month with `from_format()`
+
 ### Fixed
 
 - Fixed `from_format()` not recognizing input strings when the specified pattern had escaped elements.
 - Fixed missing `x` token for string formatting.
 - Fixed reading timezone files.
-- Added support for parsing padded 2-digit days of the month with `from_format()`
 - Fixed `from_format()` trying to parse escaped tokens.
 - Fixed the `z` token timezone parsing in `from_format()` to allow underscores.
 - Fixed C extensions build errors.
diff --git a/pendulum/duration.py b/pendulum/duration.py
index 54d113d3..df0ef16b 100644
--- a/pendulum/duration.py
+++ b/pendulum/duration.py
@@ -9,6 +9,7 @@
 from pendulum.utils._compat import decode
 
 from .constants import (
+    DAYS_PER_WEEK,
     SECONDS_PER_DAY,
     SECONDS_PER_HOUR,
     SECONDS_PER_MINUTE,
@@ -78,7 +79,8 @@ def __new__(
         if total < 0:
             m = -1
 
-        self._microseconds = round(total % m * 1e6)
+        # round returns a float in python2, so ensure stored as an int
+        self._microseconds = int(round(total % m * 1e6))
         self._seconds = abs(int(total)) % SECONDS_PER_DAY * m
 
         _days = abs(int(total)) // SECONDS_PER_DAY * m
@@ -243,6 +245,58 @@ def _sign(self, value):
 
         return 1
 
+    def to_iso8601_string(self):
+        """
+        Return the duration as an ISO8601 string.
+        
+        Either:
+            "PnYnMnDTnHnMnS"
+            "PnW" - if only weeks are present
+        """
+        rep = "P"
+        if self._years:
+            rep += "{}Y".format(self._years)
+        if self._months:
+            rep += "{}M".format(self._months)
+        # days without any specified years, months
+        days_alone = self._days - (self.years * 365 + self.months * 30)
+        if days_alone:
+            rep += "{}D".format(days_alone)
+        time = "T"
+        if self.hours:
+            time += "{}H".format(self.hours)
+        if self.minutes:
+            time += "{}M".format(self.minutes)
+        # TODO weeks
+        # TODO signs and test
+        s = ""
+        if self.remaining_seconds:
+            s = str(self.remaining_seconds)
+        if self.microseconds:
+            # no division to avoid possible floating point errors
+            if not s:
+                s = "0"
+            s += ".{:0>6d}".format(self.microseconds).rstrip("0")
+        if s:
+            time += "{}S".format(s)
+        if len(time) > 1:
+            rep += time
+        if len(rep) == 1:
+            # 0 duration
+            rep = "PT0S"
+        # check if PnW representation is suitable
+        # i.e. only days
+        # TODO abs?
+        if (
+            days_alone % DAYS_PER_WEEK == 0
+            and not self._years
+            and not self._months
+            and len(time) == 1
+        ):
+            w = days_alone // DAYS_PER_WEEK
+            rep = "P{}W".format(w)
+        return rep
+
     def as_timedelta(self):
         """
         Return the interval as a native timedelta.
diff --git a/tests/duration/test_to_iso8601.py b/tests/duration/test_to_iso8601.py
new file mode 100644
index 00000000..06bee8cd
--- /dev/null
+++ b/tests/duration/test_to_iso8601.py
@@ -0,0 +1,114 @@
+import pytest
+
+import pendulum
+
+
+def test_all():
+    d = pendulum.duration(
+        years=2, months=3, days=4, hours=5, minutes=6, seconds=7, microseconds=50
+    )
+
+    expected = "P2Y3M4DT5H6M7.00005S"
+    assert d.to_iso8601_string() == expected
+
+
+def test_basic():
+    d = pendulum.Duration(years=2, months=3, days=4, hours=5, minutes=6, seconds=7)
+
+    expected = "P2Y3M4DT5H6M7S"
+    assert d.to_iso8601_string() == expected
+
+
+def test_microsecond_alone():
+    d = pendulum.duration(microseconds=5)
+
+    expected = "PT0.000005S"
+    assert d.to_iso8601_string() == expected
+
+
+def test_microsecond_trailing_zeros():
+    d = pendulum.duration(microseconds=500)
+
+    expected = "PT0.0005S"
+    assert d.to_iso8601_string() == expected
+
+
+def test_second_and_microsecond():
+    d = pendulum.duration(seconds=50, microseconds=5)
+
+    expected = "PT50.000005S"
+    assert d.to_iso8601_string() == expected
+
+
+def test_lots_of_days():
+    d = pendulum.duration(days=500)
+    # should not be coverted to months, years as info is lost
+
+    expected = "P500D"
+    assert d.to_iso8601_string() == expected
+
+    d = pendulum.duration(days=40)
+
+    expected = "P40D"
+    assert d.to_iso8601_string() == expected
+
+
+@pytest.mark.skip(reason="This test will fail until large changes to normalization")
+def test_lots_of_hours():
+    # NOTE: this will fail until total_seconds normalization
+    # no longer occurs
+    d = pendulum.duration(hours=36)
+    # should not be coverted to days, as can be different
+    # depending on daylight savings changes
+
+    expected = "PT36H"
+    assert d.to_iso8601_string() == expected
+
+
+def test_days_and_months():
+    d = pendulum.duration(months=1, days=40)
+    # not equivalent to P2M10D
+
+    expected = "P1M40D"
+    assert d.to_iso8601_string() == expected
+
+
+def test_weeks_alone():
+    d = pendulum.duration(days=21)
+
+    # Could also validly be P21D
+    expected = "P3W"
+    assert d.to_iso8601_string() == expected
+
+
+def test_weeks_and_other():
+    d = pendulum.duration(years=2, days=21)
+
+    expected = "P2Y21D"
+    assert d.to_iso8601_string() == expected
+
+
+def test_weeks_and_time():
+    d = pendulum.duration(days=21, minutes=7)
+
+    expected = "P21DT7M"
+    assert d.to_iso8601_string() == expected
+
+
+def test_empty():
+    # NOTE: can't validly test this in isolation,
+    # as "P0D" and "PT0S" etc. are equally valid
+    # ISO8601 representations
+    d = pendulum.duration()
+    s = d.to_iso8601_string()
+    # should be something like "PT0S" or "P0D"
+    parsed = pendulum.parse(s)
+
+    assert parsed.years == 0
+    assert parsed.months == 0
+    assert parsed.weeks == 0
+    assert parsed.remaining_days == 0
+    assert parsed.hours == 0
+    assert parsed.minutes == 0
+    assert parsed.remaining_seconds == 0
+    assert parsed.microseconds == 0
diff --git a/tests/parsing/test_parse_iso8601.py b/tests/parsing/test_parse_iso8601.py
index 3d2c3ea6..00bce1f5 100644
--- a/tests/parsing/test_parse_iso8601.py
+++ b/tests/parsing/test_parse_iso8601.py
@@ -160,7 +160,7 @@ def test_parse_ios8601_invalid():
         parse_iso8601("2012-W123")  # Missing separator
 
 
-def test_parse_ios8601_duration():
+def test_parse_iso8601_duration():
     text = "P2Y3M4DT5H6M7S"
     parsed = parse_iso8601(text)
 
diff --git a/tests/test_parsing.py b/tests/test_parsing.py
index 4f50d863..b1236ea8 100644
--- a/tests/test_parsing.py
+++ b/tests/test_parsing.py
@@ -86,6 +86,20 @@ def test_parse_duration():
     assert isinstance(duration, pendulum.Duration)
     assert_duration(duration, 0, 0, 2, 0, 0, 0, 0)
 
+    text = "PT0S"
+
+    duration = pendulum.parse(text)
+
+    assert isinstance(duration, pendulum.Duration)
+    assert_duration(duration, 0, 0, 0, 0, 0, 0, 0)
+
+    text = "P0D"
+
+    duration = pendulum.parse(text)
+
+    assert isinstance(duration, pendulum.Duration)
+    assert_duration(duration, 0, 0, 0, 0, 0, 0, 0)
+
 
 def test_parse_interval():
     text = "2008-05-11T15:30:00Z/P1Y2M10DT2H30M"