The structure of the title solvate, C 9H 6O 6·C 2H 6OS, was determined 30 years ago [Herbstein, Kapon & Wasserman (1978 ▶). Acta Cryst. B 34, 1613–1617], with data collected at room temperature, and refined in the space group P2 1. The present redetermination, based on high-resolution diffraction data, shows that the actual space group is more likely to be P2 1/ m. The crystal structure contains layers of trimesic acid molecules lying on mirror planes. A mirror plane also passes through the S and O atoms of the solvent molecule. The molecules in each layer are interconnected through strong O—H⋯O hydrogen bonds, forming a two-dimensional supramolecular network within each layer. The donor groups are the hydroxyls of the trimesic acid molecules, while the acceptors are the carbonyl or the sulfoxide O atoms.