The influence of a bias electric field on the phase velocity of acoustic layer modes propagating along a ZnO overlay on a Si substrate is analyzed. Measurements of the fractional change in phase velocity as a function of the applied electric field were performed on both Rayleigh and Sezawa modes. The strength of the electroelastic effect has been experimentally determined for two samples obtained by different sputtering runs. An evident dependence of this parameter on the acoustic mode and on the quality of the sputtered film was observed. The maximum value of 15.8×10−6&mgr;m/V was attained by the Sezawa wave in one of the specimens, while a weaker effect was observed in the other cases.