Abstract The surface chemistry of methyl iodide on Ru(001) has been investigated by temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and high resolution electron energy loss spectroscopy (HREELS). CH 3I adsorbs dissociatively at 110 K, and, during TPD, yields hydrogen at low exposures and both hydrogen and methane at high exposures. In contrast to other metals, HREELS shows that CC bonds are formed during thermally activated surface rearrangement. Surface reaction intermediates methyl (CH 3(a)), ethylidyne (CH 2(a)), methylene (CCH 3(a)), and methylidyne (CH(a)) are identified by HREELS. A decomposition mechanism is proposed.