Selectively deuterium-labeled isotopomers of 1,3-butadiene (CD2CDCDCD2, CD2CHCHCD2, and CH2CDCDCH2) have been used as feed reactants to study kinetic isotope effects (KIEs) in the epoxidation of butadiene over both unpromoted and Cs-promoted catalysts. On unpromoted catalysts, the rate of formation of the epoxide product, 3,4-epoxy-1-butene (EpB), is increased relative to that for unlabeled butadiene for the butadiene isotopomers labeled on the terminal carbon atoms. This indicates that EpB formation is characterized by an inverse KIE that is caused by a primary KIE in the rate of the combustion reaction and also demonstrates that combustion is initiated by cleavage of a terminal C-H bond. These KIE phenomena can be explained by a reaction mechanism in which both partial and complete oxidation products are produced from a common C4H6O(ads) surface intermediate. The measured KIEs for the reaction over the Cs-promoted catalyst are also consistent with this mechanism. Comparisons between KIE measurements in ethylene and butadiene epoxidation are discussed, and links with surface science studies Of C4H6O oxametallacycle reactions are established.