The reaction of formaldehyde has been studied on the surfaces of UO2 (1 1 1) single crystal and polyerystalline UO2. Most of adsorbed formaldehyde on the single crystal at room temperature reacted to give ethylene during TPD. On the polycrystalline surface, IR analyses showed that the main species present in the 88-200 K temperature domain are poly- and dioxymethylene. These species are mainly converted to stable formates that decompose at high temperature (above 550 K). The apparent activation energy for the reaction of the oxymethylenic species to formates in the 250-450 K region (calculated from the intensity of the v(a) COO IR band of formates, at 1575 cm(-1)) is found equal to 12 kJ/mol. Overall, both surfaces (single crystal and polycrystalline) give qualitatively the same products, with the exception of formation of traces of methyl formate in the case of powder. However, the single crystal is far more active for the reductive coupling of formaldehyde to ethylene than the polycrystalline material. The difference in reaction selectivity between the single crystal and powder work is attributed to the method of preparation of the polycrystalline UO2 material (made by H-2-reduction of alpha-U3O8) rather than to an intrinsic difference between both materials. (C) 2003 Published by Elsevier B.V.