Rates 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 for p-methyl benzyl chloride in various acetone-water mixtures. Measurements have also been made of the partial molar volumes of the reactants. The plots of log k against pressure are fitted to a second-degree polynomial in P, and values of AVt and (aAV:/aP)p are obtained. The AVZ values are all negative, having values ranging from -18 to -24 cclmole. The results are interpreted on the view that the mechanisms are SN2(1), i.e. are towards the SNl end of the SN2 spectrum of behavior. The AV: values steadily become more negative in the series p-CH,, H, p-CI, p-NO,, and this is interpreted in terms of the greater spreading of positive charge in the P-CH, case and in terms of greater SN2(2) character in the p-NO, case. The A V: 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 (aAV:/aP)p values show a negative correlation with AV:, suggesting, as expected, that the more compact activated complexes are the least compressible.