The present paper summarizes theoretical investigations of geometry and electronic structure of (oxo)carbide, whose formation is modeled by systematic replacement of lattice oxygen atoms in MoO 3 by carbon atoms or by CH 2 groups. Both, in bulk and in the small surface cluster, the formation of molybdenum (oxo)carbide is accompanied by by-products observed in experiment, namely C 2 species and CO. The present theoretical studies reveal that these are formed without reaction barrier, even though in bulk the atom mobility is limited. The thermodynamic considerations based on the obtained DFT results indicate that the process of MoO 3 reduction to yield (oxo)carbides is endoenergetic and there is no synergy between the amount of carbon already introduced to the system and the energetic cost of replacing oxygen atoms by CH 2 .