diff --git a/Lib/datetime.py b/Lib/datetime.py
index 952aebfdec0a7c..2294ac2b6873a5 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -1452,7 +1452,8 @@ def isoformat(self, timespec='auto'):
         part is omitted if self.microsecond == 0.
 
         The optional argument timespec specifies the number of additional
-        terms of the time to include.
+        terms of the time to include. Valid options are 'auto', 'hours',
+        'minutes', 'seconds', 'milliseconds' and 'microseconds'.
         """
         s = _format_time(self._hour, self._minute, self._second,
                           self._microsecond, timespec)
@@ -1937,7 +1938,8 @@ def isoformat(self, sep='T', timespec='auto'):
         time, default 'T'.
 
         The optional argument timespec specifies the number of additional
-        terms of the time to include.
+        terms of the time to include. Valid options are 'auto', 'hours',
+        'minutes', 'seconds', 'milliseconds' and 'microseconds'.
         """
         s = ("%04d-%02d-%02d%c" % (self._year, self._month, self._day, sep) +
              _format_time(self._hour, self._minute, self._second,
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index acdde83dc845a5..4e0c3783d91eb8 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -4673,7 +4673,10 @@ static PyMethodDef time_methods[] = {
     {"isoformat",   (PyCFunction)(void(*)(void))time_isoformat,        METH_VARARGS | METH_KEYWORDS,
      PyDoc_STR("Return string in ISO 8601 format, [HH[:MM[:SS[.mmm[uuu]]]]]"
                "[+HH:MM].\n\n"
-               "timespec specifies what components of the time to include.\n")},
+               "The optional argument timespec specifies the number "
+               "of additional terms\nof the time to include. Valid "
+               "options are 'auto', 'hours', 'minutes',\n'seconds', "
+               "'milliseconds' and 'microseconds'.\n")},
 
     {"strftime",        (PyCFunction)(void(*)(void))time_strftime,     METH_VARARGS | METH_KEYWORDS,
      PyDoc_STR("format -> strftime() style string.")},
@@ -6380,9 +6383,10 @@ static PyMethodDef datetime_methods[] = {
                "YYYY-MM-DDT[HH[:MM[:SS[.mmm[uuu]]]]][+HH:MM].\n"
                "sep is used to separate the year from the time, and "
                "defaults to 'T'.\n"
-               "timespec specifies what components of the time to include"
-               " (allowed values are 'auto', 'hours', 'minutes', 'seconds',"
-               " 'milliseconds', and 'microseconds').\n")},
+               "The optional argument timespec specifies the number "
+               "of additional terms\nof the time to include. Valid "
+               "options are 'auto', 'hours', 'minutes',\n'seconds', "
+               "'milliseconds' and 'microseconds'.\n")},
 
     {"utcoffset",       (PyCFunction)datetime_utcoffset, METH_NOARGS,
      PyDoc_STR("Return self.tzinfo.utcoffset(self).")},