Solvent effects on the keto–enol equilibria of ethyl acetoacetate, acetylacetone, ethyl cyclopentanone-2-carboxylate, and methyl 4-methylcyclopentane-1-,2-dione-3,4,5-tricarboxylate have been studied by ultraviolet spectroscopy. The extent of enolization is mainly determined by the stabilization of the keto form by local association with polar or proton-donating solvent molecules, just as in the case ofn → π* transitions and infrared stretching frequencies. Solvent effects on infrared spectra reveal useful information regarding the characteristic frequencies of the tautomers.