2-{2-[4-(dibenzo[b,f][1,4]thia-zepin-11-yl)piperazin-1-yl 1-oxide]eth-oxy}ethanol-fumaric acid (2/1)), C(21)H(25)N(3)O(3)S·0.5C(4)H(4)O(4), is one of the oxidation products of quetiapine hemifumaric acid. In the tricyclic fragment, the central thia-zepine ring displays a boat conformation and the benzene rings are inclined to each other at a dihedral angle of 72.0 (2)°. The piperazine ring adopts a chair conformation with its eth-oxy-ethanol side chain oriented equatorially. In addition to the main mol-ecule, the asymmetric unit contains one-half mol-ecule of fumaric acid, the complete mol-ecule being generated by inversion symmetry. In the crystal, O-H⋯O hydrogen bonds link the components into corrugated layers parallel to bc plane.