Transition metal carbides (TMCs) are of increasing interest for catalytic processes. Their performance and stability under common oxidative conditions in catalytic reactions are crucial for several applications, including catalysis and electrochemical reactions. In this work, we report a detailed XPS study of the interactions of stoichiometric NbC and TaC surfaces with common oxidizing agents like O2 and H2O, which are important media in many chemical processes. Experimental results showed that NbC reacts with O2 to produce Nb sub-oxrides, while TaC is inert to O2 exposure. TaC surfaces are more sensitive to H2O vapor, with a greater surface oxidation and hydroxylation. Atmospheric oxidation of NbC and TaC was also studied, and results showed that both films oxidized yielding to the formation of Nb2O5 and Ta2O5, hydroxylated/ oxide carbon species, and some adventurous carbon build-up. TMCs are catalytically active in many reactions, especially those involving electrochemical nitrogen reduction reactions (NRR) to ammonia. Experimental and DFT calculations were used to provide insight on how carbide surface structures change electrochemically and how that evolution relates to NRR activity. Results showed that NbC has NRR activity at pH 3.2 after immersion in 0.3 M NaOH, leaving niobium suboxides. However, photoemission data showed that the Nb2O5 overlayer is restored after polarization to -1.3 V vs. Ag/AgCl, inhibiting NRR activity. TaC, on the other hand, is inactive for NRR at potentials more positive than -1.0 V, as NaOH treatment fails to remove the Ta2O5 surface layer induced by ambient exposure. The study also found that the formation and stabilization of intermediate oxidation states on the surface of transition metal ions are crucial for N≡N bond activation and NRR activity.