A single-crystal structure determination has revealed that the product obtained by oxidation of quinol with sodium chlorate and osmium tetroxide and crystallized from dimethyl sulfoxide is a solvate of 1α,4aβ,6β,9aα-tetrahydroxy-4a,5aα,9a,10aβ-tetrahydrodibenzo[b,e][1,4]dioxin-2,7(1H,6H)-dione. The crystals of C₁₂H₁₂O₈,2(CH₃)₂SO are monoclinic, P2₁/n, a 7.859(5), b 9.774(7), c 13.14(1) Ǻ, β 93.08(6)°, Z 2, with each molecule of the dione centred on a crystallographic centre of symmetry. The solvent molecules are disordered. The structure was refined by least squares to a residual of 0.046 for 1361 'observed' reflections.