An algorithm of calculation of equilibrium composition of complex chemically reacting systems at high pressure and temperature is described. The algorithm, which was used for the development of a computer code, may be applied for examination of various classes of thermodynamic systems, but its principal area of applicability is analysis of combustion products. Two real gas equations of state (virial with three coefficients and equation of state, proposed by V.I. Nedostup) are incorporated into the code. Results of rest calculations presented demonstrates the capabilities of the program and confirms the reliability of the data obtained.