In this study, we observed that the ferroelectric properties of (Pb1−xLax)TiO3, (PLT) thin films deposited on Pt/Ti/Si substrates using SrRuO3 as a buffer layer change markedly with the substrate temperature and the composition. All the films are perovskite with no secondary phases when deposited at 500–600 °C. However, only the films deposited below 520 °C possess a satisfactory small leakage current density, for example, JL ⩽ 10−7 A/cm2, under a 50 kV/cm applied field. Both PLT10 (x = 0.10) and PLT5 (x = 0.05) thin films thus obtained possess large pyroelectric coefficient (p = 0.009–0.018 °C−1). However, the PLT10 thin films show pyroelectric properties markedly superior to the PLT5 thin films, although the PLT5 thin films own much better ferroelectric properties. This phenomenon is explained by the lower Curie temperature (Tc) of the PLT10 materials. © 1998 American Institute of Physics.