As far as frequency is lower than the mechanical resonance, all electrooptic (EO) index modulations contain a contribution from the elastooptic effect induced by thickness modulations (piezoelectricity or electrostriction). Waveguide EO modulators made of polymer films are designed for very high frequencies (above the mechanical resonance≈1 GHz), but EO measurements are performed at low frequency. In this paper two methods are presented for measuring the elastooptic properties of polymer films. The first is a direct measurement of the acoustooptic modulation of the index of refraction. The second is based on the comparison of the EO effect with two different configurations of electrodes, producing different mechanical modulations. Results are shown to be in reasonable agreement with a simple model which can be used routinely for estimating the correction of EO measurements.