The transition structures and a-carbon 12C/13C kinetic isotope for 22 SN2 reactions between methyl chloride and a wide variety of nucleophiles have been calculated using the B1LYP/aug-cc-pVDZ level of theory. Anionic, neutral, and radical anion nucleophiles were used to give a wide range of SN2 transition states so the relationship between the magnitude of the a-carbon kinetic isotope effect and trasition-state structure could be determined. The results suggest that the a-carbon 12C/13C kinetic isotope effects for SN2 reactions will be large (near the experimental maximum) and that the curve relating the magnitude of the KIE to the percent transfer of the a-carbon from the nucleophile to the leaving group in the transition state has a broad maximum. This means very similar KIEs will be found for early, symmetric, and late transition states and that one cannot use the magnitude of these KIEs to estimate transition-state structure.