The rate of the uncatalyzed chlorination of toluene by molecular chlorine was studied without added solvent and in acetic acid solution. During competitive reaction in acetic acid, the chlorination Of C7H8 proceeded at a rate equal to that of C7D8. The rate of reaction was estimated to be smaller by a factor of 10(-4) when no solvent was added. Calculations using density functional theory suggest that the reaction is bimolecular and does not involve an arenium cation as an intermediate when low-dielectric solvents are used. The carbon-hydrogen bond is broken late in the reaction process, consistent with the observed kinetic isotope effect. The difference between the reaction rate without solvent and that in acetic acid can be attributed to the stabilization of the activated complex in acetic acid solution. The computational results suggest that in other solvents, with higher dielectric constants, the accepted reaction pathway involving an arenium cation is likely to predominate.