AbstractRates of solvolysis of benzyl chloride and of substituted benzyl chlorides have been measured in an acetone‐water mixture (acetone mole fraction 0.147) at pressures ranging from atmospheric to 1 kbar. Pressure studies have also been made forp‐methyl benzyl chloride in various acetone‐water mixtures. Measurements have also been made of the partial molar volumes of the reactants. The plots of logkagainst pressure are fitted to a second‐degree polynomial inP, and values of ΔV‡and (δΔV//δP)Tare obtained. The ΔV‡values are all negative, having values ranging from −18 to −24 cc/mole. The results are interpreted on the view that the mechanisms areSN2(1), i.e. are towards theSN1 end of theSN2 spectrum of behavior. The ΔV‡values steadily become more negative in the seriespCH3, H,pCl,pNO2, and this is interpreted in terms of the greater spreading of positive charge in thepCH3case and in terms of greaterSN2(2) character in thepNO2case. The ΔV‡values go through a minimum as the solvent composition is varied, a result that is related to the existence of a corresponding maximum in the partial molar volumes of the reactant. The (δΔV‡/δP)Tvalues show a negative correlation with ΔV‡, suggesting, as expected, that the more compact acti