To determine the phase relationships in the system (1 - 2x)BiSc03 • xPbTi03 • xPbMg1/3Nb2/303 near the morphotropic boundary, the ceramic samples for x = 0.34 and x = 0.46 were studied by the XRD method. The profiles of nine regions of the XRD patterns were analyzed using WinFit software. In both cases, a much better agreement with the experimental data was observed with the introduction of additional phases with cubic symmetry. The simulation of the full x-ray diffraction patterns of the samples using the Le Bail method was also carried out. The studied samples contain two phases: the main with a cubic (o = 4.0432 A) (x = 0.34) and tetragonal (o = 3,9963 А, с = 4.0580 A) (x = 0.46) unit cells and additional one. The additional phases with broad diffraction peaks can be considered cubic (with the unit cell parameters a - 4.045 A and a - 4.017 A, respectively).