Abstract Decomposition of methanol on the clean and oxygen-modified Fe(100) surfaces has been studied, using high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). The Fe(100) surface was modified by adsorption of O 2 at 113 K, heating to 923 K, followed by cooling back to 113 K. Chemisorbed O atoms following this treatment are located on four-fold hollow sites. These preadsorbed O atoms selectively poison the decomposition of methanol into CO, which dominates on the clean Fe(100) surface, and open up a new reaction pathway which results in the formation of formaldehyde. The decomposition of CH 3OH involves a stable methoxy intermediate for both decomposition products. The thermal stability of the methoxy increases as the O coverage is increased. The presence of preadsorbed four-fold hollow O atoms reduces the amount of methoxy which can be formed on the surface, poisoning the surface for the decomposition of methanol. A mechanism for the decomposition reaction on clean and modified Fe(100) surfaces which accounts for these observations is proposed.