On the basis of a lattice-gas model, a technique for calculating phase diagrams has been extended to mixed species on heterogeneous surfaces. It incorporates three stages : (1) a total List of all possible phases where an adsorption system can be is written down on the basis of lattice symmetry and a range of lateral interactions; (2) a diagram of ground states is constructed at zero temperature on the basis of the listed phases; (3) phase boundaries are determined by calculating adsorption isotherms. An analysis of the diagrams of ground states for heterogeneous regular and patchwise surfaces is carried out. On the regular surfaces, phase transitions occur on a whole surface, while on the patchwise ones each homogeneous patch works independently with its own contribution to the summary diagram of the ground states. Moreover, the regular surface facilitates formation of the ordered phases. Adsorption isotherms and desorption rate dependencies on concentration were studied allowing for a phase transition. As an example, the CO/K/Ni(110)thermal desorption spectra were described and good agreement with experimental data was obtained.