  We discuss the role of ambipolar diffusion for simple reconnection in a
partially ionized gas, following the reconnection geometry of Parker and Sweet.
When the recombination time is short the mobility and reconnection of the
magnetic field is substantially enhanced as matter escapes from the
reconnection region via ambipolar diffusion. Our analysis shows that in the
interstellar medium it is the recombination rate that usually limits the rate
of reconnection. We show that heating effects can reduce this rate by
increasing the recombination time and raising the local ion pressure. In the
colder parts of the ISM, when temperatures are or the order of 100K or less, we
obtain a significant enhancement over the usual Sweet-Parker rate, but only in
dense molecular clouds will the reconnection velocity exceed 0.001 times the
Alfv\'en speed. The ratio of the ion orbital radius to the reconnection layer
thickness is typically a few percent, except in dense molecular clouds where it
can approach unity. We briefly discuss prospects for obtaining much faster
reconnection speeds in astrophysical plasmas.
