Solutions of benzenesulfonic acid (BSA) in acrylonitrile in the range 1.02-6.53 mol dm(-3) were studied by FT-Raman spectroscopy. Spectra in the region of the acid SOH and benzenesulfonate anion SO3 stretching bands were analysed by band-fitting procedures in order to ascertain the degree of acid dissociation. This parameter changes from 0.42 (1.02 M solution) to 0.185 (6.53 M solution) despite the strong character of the acid. Interaction of acrylonitrile with undissociated BSA produces a new band in the nu(C[triple bond]N) Raman spectral region, displaced +21.4 cm(-1) and assigned to acrylonitrile molecules H-bonded to BSA. This displacement is in accord with the strong H-donor character of the acid. From the concentration of H-bonded acrylonitrile, the mean number of H-bonds in which each solvent molecule participates can be calculated. This number changes from ca. 0.2 in the less concentrated solution up to ca. 2.4 in the most concentrated solution. These results suggest that BSA-acrylonitrile complexes of fixed stoichiometry do not occur in the range of concentrations studied here.