[en] α secondary deuterium isotope effects have been measured for the observed rate constants for addition of cyanide ion to a series of substituted benzaldehydes in aqueous solution at 25⁰C. Under the experimental conditions employed, these reactions did not proceed to completion, and the observed isotope effects were corrected to account for the influence of the reverse reaction employing measured equilibrium constants for cyanohydrin formation and previously determined secondary deuterium isotope effect for the equilibrium constant for 4-methoxybenzaldehyde cyanohydrin formation. In the four cases studied, values of k/sub D//k/sub H/ varied from 1.15 to 1.20, only slightly lower than the calculated maximal value for complete formation of the anionic tetrahedral species which is the immediate product of the rate-determining step, 1.21. A trend in isotope effect as a function of substrate reactivity could not be definitively established. The results suffice to establish that addition of cyanide to benzaldehydes proceeds via transition states in which rehybridization of carbonyl carbon to the tetrahedral geometry is nearly complete