The previously proposed order parameter functional method is considered for application to phase equilibria in ordering nonstoichiometric interstitial compounds MXy. Equations are derived for calculating the ranges of temperature and composition where a disordered phase and ordered phases exist. The ordering was taken into account for the first time to calculate the phase diagrams of the Zr-C, Hf-C, Nb-C and Ta-C systems, and the possible types of nonstoichiometric ordered carbide phases were determined. It is shown that superstructures of the M4X3 and M8X7 type cannot be formed in nonstoichiometric MXy compounds.