Formaldehyde oxidation to carbon oxides in conjunction with methanol oxidation was studied to improve understanding about the selectivity of the partial oxidation of methanol to formaldehyde. The good selectivity to formaldehyde over bulk molybdenum trioxide and iron molybdate is attributed to water, a reaction product, and methanol, which retards the further oxidation of formaldehyde. Competitive adsorption experiments monitored by IR indicate that CH2O, CH3OH, and H2O compete for the same site. The loss of the formaldehyde selectivity over the silica-supported catalyst in methanol oxidation can not be attributed to the further oxidation of gaseous formaldehyde.