Two Mo/HZSM-5 catalyst series have been tested in the partial oxidation of methane to formaldehyde at atmospheric pressure. Conventional aqueous impregnation with ammonium heptamolybdate was used for preparing catalysts with Mo loading ranging from 0.3 to 3.3 wt.%. Two different molybdenum species were detected in these catalysts: tetrahedral monomeric Mo(VI) species and octahedral coordinated polymolybdate species. For molybdenum loading higher than 1.3 wt.%, poly molybdate species are predominant causing a decrease in the dispersion and a partial blocking up of the HZSM-5 zeolite channels. A second catalyst series were prepared from HZSM-5 zeolite by aqueous impregnation and subsequent washing with an aqueous ammonia solution, yielding Mo/HZSM-5 catalysts with Mo contents ranging from 0.08 to 0.45 wt.%. A well-dispersed molybdenum oxide phase mainly constituted of tetrahedral monomeric species joined to the zeolite surface was characterized. Both catalyst series show different activity in methane conversion. Polymolybdate species of impregnated Mo/HZSM-5 catalysts are more reactive in methane activation than acid sites of HZSM-5 but lead to low HCOH selectivity values. In contrast, treated catalysts show lower methane conversion but higher HCOH selectivity. These results demonstrate the specificity of the Mo=O sites of monomeric species for HCOH production.