The nucleophilic addition reactions of several alcohols and 1‐propanethiol to a CN double bond of 3,4‐disubstituted derivatives of 1,2,5‐thiadiazole 1,1‐dioxide were studied in acetonitrile solution. The substrates used were 3,4‐diphenyl (1a), 3‐methyl‐4‐phenyl (1b), phenanthro[9,10‐c] (1c) and acenaphtho[1,2‐c]‐1,2,5‐thiadiazole 1,1‐dioxide (1d), 3,4‐diphenyl‐1,2,5‐thiadiazoline 1,1‐dioxide (2a) and 4‐ethoxy‐5‐methyl‐3,4‐diphenyl‐1,2,5‐thiadiazoline 1,1‐dioxide (2b). Spectroscopically (UV–VIS) and electrochemically (cyclic voltammetry) measured equilibrium constants at 25.0 °C are presented and discussed for the reaction of 1a and 1b with alcohols (methanol, n‐propanol, n‐butanol, isobutanol, 2‐propanol, sec‐butanol, tert‐butanol, ethylene glycol, allyl alcohol and 2‐phenylethanol). A reaction of 1‐propanethiol with 1c was observed, but the alcohols did not react with 1c, 1d or the thiadiazolines (2a, 2b). The effect of the solvent on the equilibrium constant of the 1a–ethanol system was measured using, besides acetonitrile, propylene carbonate, N,N‐dimethylformamide, N,N‐dimethylacetamide, dimethyl sulfoxide and tert‐butanol. The results were correlated with an empirical H‐bond acceptor solvent parameter. Previously unreported spectroscopic (UV–VIS, IR, 1H and 13C NMR) data for some of the compounds studied are also provided. Copyright © 2000 John Wiley & Sons, Ltd.