The enol-keto tautomerism of N-(2-hydroxy-1-naphthylidene)anilines was studied. In response to the action of salts (e.g. CaCl2), in ethanolic solution the equilibrium is shifted extensively in the direction of the keto form; this allows determination of the concentration of the keto form and of the equilibrium constant of the process. In accordance with the Hammett equation, the equilibrium constant varies as a function of the constants a of the substituents on the aniline ring.