The partial oxidation of methane to synthesis gas over yttrium-stabilized zirconia (YSZ) was studied with in situ FTIR and both steady-state and transient experiments. The four major products, CO, H-2 CO2, and H2O, are primary products of CPOM over YSZ. Besides these major products and traces of hydrocarbons, traces of formaldehyde and formic acid were observed in the product mixture for the first time, especially at high reaction temperatures. In situ IR showed that formate was formed by activation of methane on the surface of YSZ catalysts under reaction conditions at temperatures between 400 and 475 degreesC. Adsorbed formaldehyde was never observed, due to rapid conversion to formate. Temperature-programmed desorption/decomposition (TPD/TPDE) of formate resulted in an essentially identical mixture of CO, CO2, H-2, and H2O to normal CPOM. Addition of formaldehyde to the CPOM reaction mixture caused the yields Of CO, CO2, and H-2 to increase without influencing the product distribution. This is a strong indication that CH2O is indeed an intermediate product, in agreement with the fact that traces of formaldehyde were observed in the product mixture. A reaction scheme is proposed that contains exclusively reaction pathways that contribute significantly; the ratios between the competing pathways is strongly influenced by temperature but does not vary at all when formaldehyde is added. CO and H-2 are formed via decomposition of both adsorbed formaldehyde and formate, while CO2 is produced via decomposition of formate mainly. Activation of methane is the rate-determining step at 600 degreesC and higher temperatures. (C) 2004 Elsevier Inc. All rights reserved.