The first-order phase transition in La1-xSmxGaO3 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 GdFeO3 structure type (Pbnm space group, Z = 4), whereas the symmetry of HT La0.9Sm0.1GaO3 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.