The structure of the title solvate, C(9)H(6)O(6)·C(2)H(6)OS, was determined 30 years ago [Herbstein, Kapon & Wasserman (1978 ▶). Acta Cryst. B34, 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 inter-connected through strong O-H⋯O hydrogen bonds, forming a two-dimensional supra-molecular network within each layer. The donor groups are the hydroxyls of the trimesic acid mol-ecules, while the acceptors are the carbonyl or the sulfoxide O atoms.