Kinetic isotope effect (KIE) data have been measured using O-18(2) for butadiene epoxidation over Cs-promoted, supported Ag catalysts. These show that the rate-limiting step for butadiene epoxidation is dissociation of a molecular oxygen species (O-2)(-1) on a vacant Ag surface site. Comparisons have been made between the experimentally measured KIE values and calculated KIE values for reaction steps (other than O-O dissociation) involving bond-making or bond-breaking steps in which oxygen is involved. In all these instances the calculated KIE values are much lower than the KIE actually observed. This study marks the first instance where O-18(2) has been used at steady-state olefin epoxidation conditions to confirm the nature of the oxygen active in olefin epoxidation. The O-18 results in this study also directly support the current belief that atomic oxygen, and not a molecular oxygen species, is the active form of oxygen that reacts with olefins to form olefin epoxides. Finally, comparison of the kinetics for butadiene epoxidation with the kinetics for ethylene epoxidation shows that the rate-limiting steps for the two reactions are different. For ethylene epoxidation, the surface reaction between adsorbed ethylene and adsorbed oxygen is considered to be the limiting step, while dissociation of molecular oxygen dissociation is rate limiting for butadiene epoxidation.