By means of X-ray diffraction method, we determined the lattice parameters and phase transition temperatures of specimen of K0.5Bi0.5TiO3—Na0.5Bi0.5TiO3 system with varions components, as well as the component of the boundary between tetragonal and rhombohedral forms. The X-ray powder diffraction data of K0.5Bi0.5TiO3—Na0.5Bi0.5TiO3 polycrystals are also given.