Formaldehyde adsorption and reaction have been studied on cerium oxide thin films that were vapor deposited on Ru(0001). The formaldehyde behavior was examined as a function of temperature, exposure and Ce oxidation state. Formaldehyde chemisorbs on fully oxidized CeO 2 as dioxymethylene, CH 2 O 2 . The dioxymethylene decomposes and desorbs as formaldehyde between 200K and 400K. No other products are formed. On reduced ceria, formaldehyde also adsorbs as dioxymethylene. In addition to the formaldehyde desorption between 200K and 400K, a more strongly bound form of dioxymethylene is formed that produces formaldehyde at 440K. Above 400K, some of the dioxymethylene reacts to form formate and methoxy on the surface. These species decompose to produce H 2 , CO and CH 2 O above 500K.