The catalytic performance of Mo/ZrO2 catalysts in selective oxidation of methane to formaldehyde with molecular oxygen as oxidant has been investigated. The maximal yield of formaldehyde, ca. 4.0% (47.8% formaldehyde selectivity), was obtained on 12 wt.% Mo/ZrO2 catalyst at 400 degreesC, 5.0 MPa, CH4/O-2/N-2 = 10/1/3 and 12,000 ml g(-1) catalyst h(-1). The physicochemical properties of these catalysts, such as BET surface area, structure, particle size, reducibility, ion oxidation state and surface composition have been comparatively characterized by using BET, X-ray diffractometer (XRD), LR spectra (LRS), H-2-temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) technology. The results clearly show that some interaction between Mo and ZrO2 occurred. Such interaction induced the changes of physicochemical properties, which in turn determined the catalytic performance. The effects were a function of the Mo-loading. Zr(MoO4)(2) in Mo/ZrO2 catalysts were closely related to the formation of formaldehyde. The methane conversion and formaldehyde selectivity has been also correlated to the density of molybdenum oxides. The higher the density of molybdenum oxide, the higher was the specific activity. The Mo=O species of Zr(MoO4)2 enable selective oxidation of methane to formaldehyde, while the many more lattice oxygen species and bulk MoO3 accelerated the deep oxidation of product formaldehyde. (C) 2003 Elsevier Science B.V. All rights reserved.