The product distribution detected in the multimass temperature-programmed desorption of a saturated overlayer of ethylene oxide adsorbed on Pd(111) at approximately 180 K indicates that it decomposes to yield ethylene and acetaldehyde. These observations are interpreted by postulating that ethylene oxide reacts to form an oxymetallocycle. This is proposed to thermally decompose in a manner analogous to carbometallocycles that form during olefin metathesis catalysis by the reaction between an alkene and a surface carbene. Thus, the metallocycle can decompose to yield ethylene and deposit adsorbed atomic oxygen or undergo a beta-hydrogen transfer to form acetaldehyde.