The surface active sites of a fresh Mo₂C/Al₂O₃ catalyst and their evolution under passivation conditions were characterized by IR spectroscopy using CO as the probe molecule. It was found that adsorption properties of CO on the fresh sample were quite different from those of the reduced passivated one. Mo^δ+ (0 < δ < 2) sites are mainly present on fresh Mo₂C/Al₂O₃ catalyst as probed by a characteristic IR band at 2054 cm⁻¹ of adsorbed CO. When the fresh Mo₂C/Al₂O₃ catalyst in the IR cell is exposed to trace amounts of O₂ or H₂O in situ at RT, the intensity of the 2075 cm⁻¹ band declines. It shows that the fresh Mo₂C/Al₂O₃ catalyst can be oxidized by trace amounts of O₂ or H₂O easily and the oxidation capacity of H₂O is weaker than that of O₂. IR spectra of adsorbed CO on a reduced Mo₂C/Al₂O₃ catalyst passivated by a 1% O₂/N₂ mixture show that two weak bands at 2180 and 2095 cm⁻¹ appear, which suggests that the passivation layer cannot be completely reduced, even by H₂-reduction at high temperatures. For reduced Mo₂C/Al₂O₃ passivated by H₂O or CO₂, IR spectra of adsorbed CO give characteristic IR bands at 2081 and 2030 cm⁻¹, which indicates that the surface Mo atoms are in a state of Mo^ϕ+ (0 < ϕ < 3). Thus, we found that the Mo₂C/Al₂O₃ passivated by H₂O or CO₂ can be regenerated by H₂ treatment at 673 K and most of the active sites can be recovered. It is too active to control the passivation extent using O₂ as an oxidant, while Mo₂C/Al₂O₃ passivated by H₂O or CO₂ can be regenerated by simple reduction with H₂.