A new theoretical and experimental approach to determination of the thermodynamic dissociation constants of a number of active pharmaceutical compounds, such as 5-substituted-1H-tetrazoles and amino acids, has been proposed. We obtained our experimental data from A (absorbance) to C/mol.dm(-3) (concentration of weak electrolyte) without either measuring pH during the experiment or using buffer solutions with a constant ionic strength. Thermodynamic dissociation constants were obtained by extrapolating the calculated dissociation constants to zero concentration of the investigated compound. We determined the previously unknown values of thermodynamic dissociation constants of three 5-substituted-1H-tetrazoles with poorly resolved spectra of the protonated and unprotonated forms.