Reactions of propene on an oxygen-modified Ag(111) surface were investigated by means of temperature-programmed reaction spectra and reflection-adsorption infrared spectroscopy (RAIRS) at 100 K. In the presence of oxygen adatoms, interactions of propene with the substrate are strengthened. The reaction between propene and oxygen adatoms (0.086 monolayer) produces not only total oxidation products (CO2 and H2O), but also partial oxidation products (CO and acetone). The formation of chemisorbed hydroxyl groups is identified by RAIRS when coadsorbed propene and oxygen adatoms are annealed to 200 K. The formation of hydroxyl is ascribed to the abstraction of methyl hydrogen by oxygen adatoms.