The design and implementation of microelectromechanical (MEMS) systems requires a sound understanding of the influence of film stress on both the ferroelectric and piezoelectric characteristics of thin films to be used for sensing and/or actuation. Experiments were conducted in which thin film samples of sol-gel derived PZT were subjected to applied biaxial stresses from -139 to 142 MPa. Films were characterized at known stress states (derived from known values of residual stress and large deflection plate theory) in terms of their ferroelectric polarization (saturated and remnant), dielectric constants, coercive field strengths, and tan δ. Results obtained indicate that domain wall motion in thin films contributes much less to the observed response than is typical for bulk PZT materials. Alternative mechanisms are proposed in an attempt to explain the discrepancies.