The reaction between ethylene and acetate species on oxygen-covered Au/Pd(100) alloy surfaces is explored using reflection-absorption infrared spectroscopy and temperature-programmed desorption (TPD). Distinctly different behavior is found for alloys with less than similar to 0.5 monolayers (ML) of gold, where the alloy contains PdaEuro'Pd bridge sites, and alloys with gold coverages above similar to 0.5 ML, where only isolated Pd sites are available. Ethylene reacts with eta(2) acetate species on alloys with low gold coverages where it is found that acetate removal rate increases linearly with gold coverage with a concomitant decrease in carbon monoxide formation, indicating that the selectivity also increases. A different acetate species forms on the isolated palladium sites present on the alloy with high gold coverages, which exhibits infrared peaks at similar to 1450 and 1590 cm(-1), as opposed to the characteristic frequency of similar to 1414 cm(-1) found for the eta(2) acetate. It is confirmed that the infrared features are due to adsorbed acetate species by using TPD, but this form of adsorbed acetate is found to be completely unreactive with ethylene, even in a relatively high background pressure of similar to 2 Torr, compared with the eta(2) acetate species that react rapidly in the presence of ethylene pressures of similar to 10(-4) Torr. The structure is assigned to a unidentate acetate species with a strong PdaEuro'O bond, in which the C=O group can interact with an adjacent gold atom and such a structure would rationalize the lack of reactivity with ethylene because ethylene insertion into the PdaEuro'O bond of this species would be prohibited. This result is surprising because previous catalytic measurements on model gold-palladium alloys under realistic conditions have shown that high-gold coverage Au/Pd(100) alloys are the most active. A possible explanation for this discrepancy is that the alloys become enriched by palladium under reaction conditions until the gold coverage is reduced below similar to 0.5 ML to yield a very catalytically active surface.