The mechanism of L-proline-catalyzed alpha-amination of 3-phenylpropionaldehyde was studied using a combination of experimental kinetic isotope effects (KIEs) and theoretical calculations. Observation of a significant carbonyl C-13 KLE and a large primary alpha-deuterium KIE support rate-determining enamine formation. Theoretical predictions of KIEs exclude the widely accepted mechanism of enamine formation via intramolecular deprotonation of an iminium carboxylate intermediate. An E2 elimination mechanism catalyzed by a bifunctional base that directly forms an N-protonated enamine species from an oxazolidinone intermediate accounts for the experimental KIEs. These findings provide the first experimental picture of the transition-state geometry of enamine formation and clarify the role of oxazolidinones as nonparasitic intermediates in proline catalysis.