The Tl 2TlOH(SO 4) 2 compound is monoclinic, a = 7.758 (3), b = 17.587 (9), c = 7.356 (3) Å, β = 119.91 (3)°, S. G. Cc, Z = 4. The structure was solved by full-matrix least square refinement to R = 0.033 from 1242 single-crystal reflections collected on an automated diffractometer. Tl + ions ensure bonding between [Tl IIIOH(SO 4) 2] ∞ sheets. These sheets may be viewed as criss-crossing TlSO 4 chains. In the  direction, the linking of Tl III is reinforced by bidentate OH groups, so the usual hexacoordination of trivalent thallium is preserved. A transition occurring at 104°C is under investigation.