The surface state and active sites of a fresh Mo2N/Al2O3 were investigated by FTIR, volumetric chemisorption, and TPD-MS techniques. It has been found that adsorption properties of CO on the fresh sample are quite different from those of the reduced passivated one. For reduced passivated Mo2N/Al2O3, the IR spectrum of adsorbed CO shows that a band at similar to 2180 cm(-1) together with two weak bands at 2100 and 2035 cm(-1) appear, suggesting that the Mo4+ cation is predominant on the surface; namely, the surface is in oxynitride form. However, for the fresh sample, adsorbed CO gives two characteristic IR bands at 2035 and 2200 cm(-1) respectively, corresponding to the adsorbed CO on the molybdenum and the nitrogen sites, forming linearly adsorbed CO and NCO species. From the band position of adsorbed CO, the surface molybdenum atoms are slightly positively charged, i.e., in a state of Mod(delta+)(0 < 6 < 2). The assignment of the band at 2200 cm(-1) to NCO species (CO adsorbed on N site) was further confirmed by TPD-MS and volumetric chemisorption. TPD-MS shows two CO desorption peaks at 373 and ca. 473 K, indicating two different CO adsorption sites on the catalyst. The volumetric chemisorption proves that the CO uptake increases significantly when the passivated sample is nitrided at 723 K and above, compared with the case of the reduced sample. These results suggest that not only Mo sites in low valences are present on the surface of fresh Mo nitride, but also nitrogen sites are present and so active that can react with CO to form surface NCO species.