The first-order phase transition in La 1 - x Sm x GaO 3 solid solution perovskite as well as crystal structures of both, room- (RT) and high-temperature (HT) phases, have been studied by means of powder diffraction technique using synchrotron radiation, DTA and DSC analyses. The RT structure of mixed crystals with x=0.10, 0.17 and 0.30 belong to the orthorhombically distorted GdFeO 3 structure type (Pbnm space group, Z=4), whereas the symmetry of HT La 0 . 9 Sm 0 . 1 GaO 3 phase is rhombohedral (R-3c space group, Z=6). Transition temperature T c from the orthorhombic to the rhombohedral structure is linearly related to x following T c =422(5)+2599(29)x as a first approximation.