The surface chemistry of ethylene is explored on model Au/Pd(100) alloy surfaces using a combination of temperature-programmed desorption and reflection–absorption infrared spectroscopy. The heat of adsorption of ethylene on the model alloy surface is found to increase monotonically with increasing palladium coverage in the alloy, from ~33kJ/mol for a completely gold-covered surface to ~80kJ/mol as the gold coverage decreases to zero. A large change in heat of adsorption is found for palladium coverages between 0 and ~0.35 monolayers, where previous studies have shown that the surface comprises exclusively isolated palladium sites. The heat of adsorption changes more slowly for higher palladium coverages, when palladium–palladium bridge sites appear. Vinyl species are identified for palladium coverages above ~0.8 MLfrom a vibrational mode at ~1120cm−1, which disappears when the sample is heated to ~250K, due to vinyl decomposition.