AbstractSolubilities are reported for psoralen and 8‐hydroxypsoralen in binary aqueous mixtures at ambient pressure and 298 K, with organic cosolvent methanol. Gibb's energies of transfer of there two compounds are calculated from their solubilities. Transfer chemical potentials of both compounds are stabilised by increasing prapartion of organic cosolvent because of their markedly hydrophobic character.Basic hydrolysis of psoralen and 8‐hydroxypsoralen in aqueous methanol mixtures was examines kinetically, yielding Gibb's energies of activation. Derived transfer parameters are used in analysis of kinetic data to describe the influence of solvent composition on initial and transition states. Derived transfer chemical potentials for a single ion such as hydroxide ion lead to an interesting comparison of the effects of extrathermodynamic assumptions, Wells estimates and TATB assumption, on the initial and transition states. The analysis comfirm the important role played by the choice of extrathermodynamic assumption in determining the overall kinetic pattern. We describe misgivings concerning procedures used by wells to derive transfer chemical potential for single i