Carbon-13 kinetic isotope effects have been measured for the solvolysis in methanol and ethanol of 1-bromo-1-phenylethane of natural isotopic abundance. Methanolysis of the bromide at 25 °C gave a kinetic isotope effect (k12/k13) of 1.0065 ± 0.0006, and ethanolysis at 45 °C gave 1.0064 ± 0.0010. These surprisingly low effects for a bond-rupture process have been interpreted in terms of a model for the transition state in which the bonding of the isotopic. carbon is strengthened by conjugation of the electron deficient center with the ring. The results are considered to provide support for the mass fragment model for evaluation of the effective mass term of the Bigeleisen expression for the theoretical calculation of kinetic isotope effects.