The effective d33 of sub-micron piezoelectric polymer films, deposited on glass substrates, has been measured using an optical interferometer. The polymers used were polyvinylidene fluoride (PVDF) and copolymers of PVDF with trifluoroethylene (TrFE). The interferometer measures movement of the polymer surface, in response to an applied electric field, and is easily able to detect motion of 0.1 nm. In this situation, the rigid glass substrate effectively clamps the soft polymer film in the lateral direction. Nevertheless, effective d33 values approaching 10 pmV-1 have been observed. One advantage of such thin films, is the ability to pole them with quite low voltages. For example a 100 nm film can be poled by 10 volts. The low voltage required to achieve poling in such thin films, means that the poling voltage can be left permanently connected, thus maintaining the piezoelectric activity. By adding a periodic bias voltage to the driving signal, it is possible to observe hysteresis in the piezoelectric coefficient, corresponding to the polarization hysteresis loop. The piezoelectric coefficient shows an abrupt change in sign at the coercive field, corresponding to the reversal of the net polarization.