Density functional theory (DFT) calculations on stoichiometric, high-symmetry clusters have been performed to model the (100) and (111) surface electronic structure and bonding properties of titanium carbide (TiC), vanadium carbide (VC), and titanium nitride (TiN). The interactions of ideal surface sites on these clusters with three adsorbates, carbon monoxide, ammonia, and the oxygen atom, have been pursued theoretically to compare with experimental studies. New experimental results using valence band photoemission of the interaction of O₂ with TiC and VC are presented, and comparisons to previously published experimental studies of CO and NH₃ chemistry are provided. In general, we find that the electronic structure of the bare clusters is entirely consistent with published valence band photoemission work and with straightforward molecular orbital theory. Specifically, V₉C₉ and Ti₉N₉ clusters used to model the nonpolar (100) surface possess nine electrons in virtually pure metal 3d orbitals, while Ti₉C₉ has no occupation of similar orbitals. The covalent mixing of the valence bonding levels for both VC and TiC is very high, containing virtually 50% carbon and 50% metal character. As expected, the predicted mixing for the Ti₉N₉ cluster is somewhat less. The Ti₈C₈ and Ti₁₃C₁₃ clusters used to model the TiC(111) surface accurately predict the presence of Ti 3d-based surface states in the region of the highest occupied levels. The bonding of the adsorbate species depends critically on the unique electronic structure features present in the three different materials. CO bonds more strongly with the V₉C₉ and Ti₉N₉ clusters than with Ti₉C₉ as the added metal electron density enables an important π-back-bonding interaction, as has been observed experimentally. NH₃ bonding with Ti₉N₉ is predicted to be somewhat enhanced relative to VC and TiC due to greater Coulombic interactions on the nitride. Finally, the interaction with oxygen is predicted to be stronger with the carbon atom of Ti₉C₉ and with the metal atom for both V₉C₉ and Ti₉N₉. In sum, these results are consistent with labeling TiC(100) as effectively having a d⁰ electron configuration, while VC- and TiN(100) can be considered to be d¹ species to explain surface chemical properties.