We present a simple and efficient method for calculating the effect of electrostriction on the solvation of molecular species. This new method introduces solvent compressibility into the standard continuum dielectric description of the solvent. It is based on a numerical grid-based algorithm for solving Poisson’s equation and is therefore applicable to arbitrary charge distributions in cavities of arbitrary shape. Application to the anisole hydrolysis reaction in supercritical water shows that the effects of solvent compression can be large; for example, compression is observed to lower the free energy barrier to reaction by as much as 14 kcal/mol. Additionally, compression effects are found to qualitatively alter the pressure dependence of the reactivity. Finally, the inclusion of compression effects is shown to bring the calculated pressure dependence of the activation barrier into significantly better agreement with experiment.