Iodides, especially CH3I, were used as modifiers in the oxidative dehydrogenation of methanol to formaldehyde over electrolytic silver catalyst. The yield of formaldehyde was as high as 93% at 98% conversion of CH3OH for the I-modified catalyst, while it was only 85% at 94% conversion without I-modification. The optimum reaction conditions were determined and the interaction between the silver surface and the iodide additive was investigated by means of XPS and SEM. The chemisorbed iodine atoms on the silver surface caused a decrease of the surface concentration of atomic oxygen species and prevented the complete oxidation of CH3OH to CO2.