The adsorption and decomposition of ethylene over a Pt{111} single crystal surface has been investigated by fast x-ray spectroscopy. At 100 K ethene displays precursor-mediated adsorption kinetics, adopting a single environment with a saturation C2H4 coverage of 0.25 ML and binding energy of 283.2 eV Thermal decomposition proceeds above 240 K via dehydrogenation to ethylidyne with an activation barrier of 57+/-3 kJmol(-1) and, preexponential factor v=1 X 10(10+/-0.5) s(-1). Site-blocking by preadsorbed SO4 reduces the saturation ethene coverage but induces a new, less reactive pi-bonded ethene species centered around 283.9 eV, which in turn decomposes to ethylidyne at 350 K. (C) 2003 American Vacuum Society.