Using a simple methodology based on the classical Bragg–Brentano X-ray geometry we have evidenced at room temperature a pseudo-cubic phase and a monoclinic (Pm) phase in thin films of PbSc1/2Nb1/2O3 (PSN) and (PbSc1/2Nb1/2O3)0.57–(PbTiO3)0.43 (PSN–PT) respectively, the latter compound corresponding to a morphotropic phase boundary (MPB) composition. Depending on the substrate and the thickness, different domains configuration are obtained. Competition between epitaxial and thermal stress induces a complex structural evolution with temperature which is different from that of ceramic samples. Indeed PSN films transform from a pseudo-cubic (triclinic) phase towards a cubic phase whereas in PSN–PT, a succession of transformations from a pure Pm phase toward a tetragonal (possibly non polar) phase is observed, a cubic phase never being reached up to 950 K.