Abstract Variations in the unit cell occupancy of TiC x O y phase are considered in terms of successive oxygen inclusion into the lattice of defect carbides TiC 0.5 − TiC 0.9. A classification of TiC x O y solid solutions formed and the concentration limits of their existence are presented. Three types of cubic ternary solid solutions are shown to occur in the system TiCO: solid solutions of interstitial oxygen inclusion into the initial carbide; solid solutions of oxygen inclusion—carbon and metal exclusion; and solid solutions of oxygen inclusion—oxygen substitution for carbon-carbon and metal exclusion. Specific points (inflection and breaks) on the plots where n i is the number of atoms per unit cell, n i = f( y) for TiC x O y , are accounted for by both variations in the type of solid solution and the possibility of atomic arrangement in the vicinity of these points. On the basis of the above scheme some experimental peculiarities of Ti carbide phase oxydation are explained.