The sensitivity of spontaneous polarization to epitaxial strains is evaluated for several perovskite ferroelectrics using the nonlinear thermodynamic theory. For BaTiO3, PbTiO3, and Pb (Zr 0.2 Ti0.8) O3 epitaxial layers subjected to large compressive in-plane strains, the predicted relative polarization changes agree with the results of first-principles calculations. This agreement shows that the strain sensitivity of polarization is predominantly governed by the linear electrostriction even at large strains. The calculations performed for Pb (Zr1-x Tix) O3 films of different compositions further demonstrate that the strain sensitivity strongly enhances near the morphotropic phase boundary, breaking the inverse correlation with the bulk polarization that was proposed earlier. © 2008 American Institute of Physics.