The tautomerism of 2-ethoxycarbonyl-3-oxoquinuclidine and 2-ethoxycarbonyl-3-oxobenzo-[b]quinuclidine was investigated by means of IR and UV spectra and potentiometric titration. It was found that tautomeric equilibrium between the ketone, enol, and internally ionized forms is realized for 2-ethoxycarbonyl-3-oxoquinuclidine, and the position of the equilibrium is determined by the aggregate state of the substance and the nature of the solvent. The internally ionized form predominates in the crystals and in polar solvents (alcohol and water), while the ketone and enol forms are the primary ones in solvents of low polarity (chloroform and dioxane), and the percentage of the ketone form is always larger than that of the enol form. In contrast to this, 2-ethoxycarbonyl-3-oxobenzo[b]quinuclidine exists practically completely as the keto form in crystals and solutions.