The partial oxidation of methane to formaldehyde using molecular oxygen as oxidant was studied over W/HZSM-5 catalysts with tungsten loading varying from 0.3% to 11.9%. The catalysts were characterized by means of XRD, UV diffuse reflectance spectroscopy, BET, TPR and TPD of ammonia. Three different types of supported species were detected : tetrahedral monomeric species, octahedral polymeric species, octahedral polymeric species and WO3 crystallites. The relative amount of each one varies significantly with the tungsten loading. At high contents, a decrease in BET areas due to the partial blocking up of the channels of the zeolite and a lessening of the number of acid sites were observed. It was found that supported tungsten oxide phase possesses active sites to partial oxidation of methane to formaldehyde. HCOH selectivity seems to be dependent on the dispersion and the nature of the tungsten oxide species in the catalyst.