Abstract: This paper is a theoretical analysis for the medium effects on alkaline hydrolysis of ethyl acetate by the scaled particle theory, according to our own experimental results. The composition of solvent is from 0% to 90% volume percentage of ethyl alcohol aqueous solution or of DMSO aqueous solution, and takes the pure water as the standard solvent. The medium effects are well predicted by the theory. Calculated values of lg(k/k^*) are the reaction rate constants and agree with experiment. The term , partial mole intenal free enthalpy in the resulting equation for the rate constants, includs the all interreaction energy of solvent molecules with solute molecules, that are, orientation energy, induction energy and dispersion energy. It will be added by electronic energy when the interreaction is from the polar molecules-ions.