The kinetics of the simultaneous addition of ethyl [2H] alcohol to trans-cinnamonitrile and the H-D exchange at C„ catalyzed by sodium ethoxide have been studied. The observed rate equation for the addition is R = A [trans-cinnamonitrile]+B, where A and B are constants which depend on the initial concentrations of olefin and base. The forward (ki) and the backward (k-1) rate constants of the nucleophilic addition step are (3.72 ± 0.51) × 10-4 1. mol-1 s-1 and (9.58 ± 1.20) × 104 s-1, respectively. The equilibrium constant for the overall addition reaction is K = (3.9 ± 0.2) × 10-21. mol-1. The H-D exchange is faster than the addition reaction and takes place via the vinyl carbanion derived from the olefin. Cis to trans isomerization occurs during the addition of ethyl [2H] alcohol to ciscinnamonitrile. Based on kinetic data, it is suggested that this isomerization takes place by an addition-elimination mechanism.