Computer speciation models often require large numbers of equilibrium constant values. For this purpose, state-of-the-art methods for experimental constant determination, best value constant selection from published values, and constant prediction based on quantitative structure–activity relationships, are evaluated through their application to the study of the protonation constants of five thioether carboxylates in aqueous solution at 37 °C and I = 150 mmol dm−3 (NaCl).