The interaction of formaldehyde with the clean and atomic oxygen-covered Cu(1 1 1) surfaces has been studied by means of cluster model density functional calculations in which Cu-22(14,8) is used to represent the perfect Cu(1 1 1) surface. The calculations point towards a eta (1)-H2CO-O orientation with the oxygen atom almost on top of a copper surface atom. The formaldehyde adsorption energy is of similar to 22-25 kJ/mol and the internal geometry of adsorbed formaldehyde is almost identical to that of the molecule in the gas-phase. The C-O bond is almost parallel to the surface and the conformation with the molecular plane normal to the surface is slightly preferred to the conformation with the molecular plane nearly parallel to the surface. A Cu-22-O model where atomic oxygen is adsorbed on a fee hollow site was used to study the co-adsorption and reaction of formaldehyde with atomic oxygen. Oxygen co-adsorption has a dramatic effect on the formaldehyde adsorption energy which is increased by similar to 50%. The calculated energy barrier for the formation of the dioxymethylene intermediate species through the H2CO + O --> H2CO2 reaction is of similar to 36 kJ/mol.