A tungsten oxynitride (WN0.59O0.24) catalyst with a specific surface area of 63.7 m(2) g(-1) has been prepared by the temperature-programmed reaction of WO3 with NH3 and characterized by elemental analysis, X-ray powder diffraction, N-2 sorption, and X-ray photoelectron spectroscopy (XPS) in order to extensively investigate the textural variation of this material caused by exposure to ambient air. No noticeable changes in the bulk structure of tungsten oxynitride prepared here are caused by the sample storage in ambient conditions for a long period of time up to 80 days. However, its N-2 BET surface area and total pore volume were found to severely decrease with increasing the period of time of exposure to air. The XPS depth profile measurements reveal that the amorphous tungsten trioxide (WO3) species is formed on the surface of tungsten oxynitride upon exposure to air, which results in the significant modification of the surface structure and composition of the oxynitride material. In contrast, no indications of other surface tungsten phases such as dioxidic, monooxidic, and oxynitridic species are detected. The overall results of this study strongly suggest that oxygen is mobile enough to diffuse into the surface or near-surface of the oxynitride lattice even in ambient conditions, leading to the surface WO3 formation.