Abstract The photoelectron-emission threshold energies were determined for aqueous and alcoholic solutions of potassium hydroxide and potassium metal. The energies correspond to the vertical ionization potentials of the solutes. The results are in accord with the fact that in alcohol-rich solutions the hydroxide ion reacts with alcohol to produce the alkoxide ion. The threshold energy for the alkoxide ion reflects the ionization potential of the alkoxide ion in a vacuum and the solvation effect. The difference in the ionization potential in a vacuum and in a solution, which is due to the solvation effect, was found to correlate with the acceptor number of the solvent.