The secondary alpha deuterium and heavy atom kinetic isotope effects found for two different S(N)2 reactions suggest that the magnitude of secondary alpha deuterium kinetic isotape effects can be determined by the length of only the shorter (stronger) reacting bond in an unsymmetrical S(N)2 transition slate rather than by the usual nucleophiIe-leaving group distance. Although this means the interpretation of these isotope effects is more complex than has been recognized, the results suggest that they can be used to determine whether an S(N)2 transition stale is symmetrical or unsymmetrical.