[en] The implications of the intervention of ion pair intermediates for the interpretation of leaving group chlorine kinetic isotope effects (KIE) in nucleophilic substitution reactions have been examined. By comparing observed and predicted chlorine leaving group isotope effects, it was found that the isotope effect probe could provide a stringent criterion against which the viability of a particular mechanistic hypothesis could be tested. This approach demonstrated that neither the classical S/sub N/1 -- S/sub N/2 hypothesis involving only a free carbonium ion intermediate nor Sneen's ion pairs hypothesis, which excludes the possibility of direct S/sub N/2 displacement, could adequately describe all of the examined reactions of p-methoxybenzyl chloride in 70 percent acetone. An analysis of the rate and product data for reactions in this system, using the generalized Winstein scheme, did provide a good quantitative description of the rate, product, and KIE data. In like manner the results from the reactions of other p-substituted benzyl chlorides suggest that the generalized Winstein scheme can describe their behavior with a more or less continuous shift of the importance of the various possible pathways as the ring substituent and reaction conditions are varied. One of the conclusions drawn from the results of the benzhydryl chloride in 70 percent acetone studies was that the chlorine leaving group KIE is insensitive to normal salt effects. This result suggests that the chlorine leaving group KIE may also provide a useful diagnostic tool for evaluation of the extent of participation by weakly nucleophilic salts in displacement reactions