Solutions of benzenesulfonic acid (BSA) and monohydrated 4-toluenesulfonic acid (PTSA) in dimethylformamide (DMF) were studied by FT-Raman spectroscopy in the concentration range 1.0–5.0 mol dm−3. Spectra in the region of the acid S—OH and benzenesulfonate anion SO3− stretching bands were analysed using band-fitting procedures in order to ascertain the degree of acid dissociation. In BSA solutions, this parameter changes from 0.34 (1.02 M solution) to 0.11 (5.01 M solution) despite the strong character of the acid. Interaction of DMF with undissociated BSA produces a new band in the ν(C′—N—C′) Raman spectral region near 866 cm−1, displaced −11.0 cm−1 and assigned to DMF molecules hydrogen-bonded to BSA. In PTSA solutions, hydrogen bonds are formed with the oxonium ion (H3O+) dissociated from the acid. In this case, the displacement observed is only −7.0 cm−1, indicating a weaker interaction. From the concentration of hydrogen-bonded DMF, the mean number of hydrogen-bonds participating in bonding with each solvent molecule can be calculated. This number changes, in BSA solutions, from ca. 0.5 in the less concentrated solutions up to ca. 1.0 in the most concentrated. This result supports the conclusion that BSA–DMF complexes of fixed stoichiometry are not present in the range of concentrations studied here. On the contrary, the oxonium ions form a stable complex of stoichiometry (DMF)2·H3O+ which does not change with the acid concentration. Copyright © 2004 John Wiley & Sons, Ltd.