The aim of those studies was to find the dependence of the lattice constants on the real compositions of the Pb1-X-YSnXMnYTe crystals grown by the Bridgman method. The nominal values of the investigated samples were: 0.29 < x < 0.80 and 0.01 < x < 0.21. The determination of the real composition have been carried out using the electron microprobe analysis, and the studies of the lattice constants were made using the X-ray powder diffractometer. Generally, the lattice constant decreases with increasing of Sn and Mn contents from a = 6.460 A for PbTe (x = 0, y = 0) to a = 6.227 A for x = 0.78 and x = 0.20. The lattice constant changes are very difficult to describe in a simply way as a function of composition. In this paper, an attempt of finding the method of determining the sample composition from the lattice constant measurements was undertaken.