Temperature evolution of the lattice parameters of relaxor PbSc1/2Nb1/2O3 (PSN) thin films have been measured and compared to the evolution of bulk PSN. In PSN films, a room temperature tetragonal polar phase transforms at higher temperature into a nonpolar tetragonal phase. Critical and Burns temperatures, associated, respectively, to the onset of the ferroelectric phase and to the nucleation of polar nanoregions have been assigned from strong anomalies in the temperature evolution of the film. These complex behaviors are explained by a quantitative study of the strains lying in the film. (C) 2008 American Institute of Physics.