The effect of CH3I on the formaldehyde synthesis from methanol over a polycrystalline silver catalyst was investigated at temperatures up to 993 K. While the conversion of both CH3OH and O-2 were reduced, the selectivity to HCHO increased and to CO2 decreased after treating the Ag catalyst with CH3I. A notable observation drawn from a long-time operation indicates the CH3I treatment strongly restrained the formation of HCOOH that had gradually increased with time-on-strearn (TOS). It must be noted that the formaldehyde yield did not increase, but that the suppression of formic acid may make the treatment interesting nevertheless. Finally, it was found that the iodide-induced change of the reaction performance was reversible. All observations may well be explained with the assumption of an iodide-induced re-population with different oxygen species on the silver surface. (c) 2004 Elsevier B.V. All rights reserved.