UV absorption spectra of a tautomeric system (1) of 3-thianaphthenone (3-TN) were measured and the spectra of individual chemical species involved were determined (Figs 1 and 2). On acidifying an alkaline solution of 3-TN practically pure enol was formed in the first stage of reaction. The tautomeric equilibrium constant [Enol]/[Ketone] was found to be 0·18 in H 2O, 2·-02 in ethanol. In aqueous solution. the p K of the enol was 8·97. and the p K of the ketone was 9·71. The rate constants of proton transfer in the ketone-anion system were 7·9 x 10 −4 sec −1 for dissociation, and 4·05 x 10 6 M −1 sec −1 for recombination. Corresponding rate constants for the enol-anion system were of the order of 10 8–10 9 M −1 sec −1 and 0·1–1 sec −1, respectively. Freshly sublimed 3-TN is in the ketone form, and tautomerizes extremely slowly in crystals or in aprotic solvents. The results explain apparent contradictions between NMR spectra of 3-TN described by different authors, and the anomalous polarographic behaviour of the compound.