The equilibrium position and the kinetics of the extensive hydration of the title compound 1 have been investigated by spectroscopic (uv and nmr) techniques. The hydrate exists exclusively as the geometrically isomeric 1,3-phthalandiols and the hydration equilibrium constant is 7.7 ± 0.5 at 25 °C. From the temperature dependence of the equilibrium position, values of the enthalpy and entropy of hydration were calculated. The reaction is general acid – general base catalyzed; the Brønsted α and β coefficients are both near 0.5. Catalysis by Zn(II) is observed. The mechanism of the reaction (which probably proceeds via rate-determining formation of the normal gem-diol followed by rapid cyclization) is discussed.An analysis of the thermodynamic data for the hydration of a variety of carbonyl compounds reveals that the degree of hydration is normally determined by both enthalpy and entropy contributions to the free energy. However, the extensive hydration of 1 is determined solely by the enthalpy term. No hydration could be detected for benzaldehyde or the other isomeric dialdehydes.