Rate constants for the solvolysis of o-methylbenzyl chloride were determined at 30$^{circ}$ and 40$^{circ}$C in aqueous ethanol mixtures under various pressures up to 1600 bar. From the rate constants, the activation parameters ${Delta}V^{ eq}$, ${Delta}{eta}^{ eq}$, ${Delta}H^{ eq}$, ${Delta}S^{ eq}$ and${Delta}G^{ eq}$ were evaluated. The values exhibit the extremum behavior at about 0.30 mole fraction of ethanol. This behavior is discussed in terms of electrostriction. To examine the reaction mechanism by Laidler and Eyring equation, we compared the rate constants with the dielectric constants of aqueous ethanol and the number of water molecule participated in the transition state. It was concluded that solvolytic reaction proceeds via $S_N$1 mechanism.