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The kinetics of decomposition of benzyltriphenylphosphonium chloride by sodium hydroxide was studied in binary aqueous solvent mixtures ranging from pure water to about 70% (v/v) of dipolar aprotic solvents such as DMSO or acetone in presence of a series of aliphatic and aromatic amines. The rate, which is directly proportional to the organic solvent content, was found to increase to a maximum at 0.1 M amine concentration and then to decrease. The small increase in rate due to the addition of amine is explained on the basis of an amine-water interaction, where the concentration of OH− ions in the solution increases slightly due to the ability of the amines to abstract protons from water molecules in the medium. However, the decrease in rate beyond 0.1 M amine concentration is explained as due to the decrease of hydroxide ion concentration by formation of a weakly ionised quaternary ammonium hydroxide. The observed activation energies, thermodynamic parameters and radii of the activated complex for all solvent compositions are computed. The effect of the bulk dielectric constant on the reaction rate was investigated in the light of the available electrostatic theories.