Abstract Ratkovics, F. and Palágyi-Fényes, B., 1984. The effect of keto-enol tautomerism on the properties of methylethylketone + benzene mixtures. Fluid Phase Equilibria, 16: 99–116. The density, viscosity, static relative permittivity, conductivity and refractive index of methylethylketone + benzene mixtures have been measured in the interval 293.15–313.15 K. Vapor-liquid equilibrium data for the same mixtures were measured between 313.15 and 333.15 K. The results obtained indicate the formation of an association complex between benzene and methylethylketone in the enolic form, and the keto-enol equilibrium is shifted accordingly towards formation of the latter. Electrolytic dissociation of the benzene-enol association complex contributes to the conductivity of the benzene-ketone mixtures. In addition to concentration, the relative permittivity also has a considerable influence on the conductivity, since a considerable number of nondissociated ion pairs are formed (in the sense of Bjerrum theory). Comparison of the model based on the physical picture outlined and the experimental results indicates that ions deriving from the benzene-enol association complex are solvated in the region of higher benzene molar ratios. The model is also suitable for the interpretation of results obtained for mixtures containing other ketones and proton acceptors.