The crystal structure of tribasic lead sulfate (3PbO • PbSO 4 • H 2 O) has been determined by a combination of single crystal X-ray and powder neutron diffraction to R of 0.024. From X-ray data, the space group is P1 , a ؍ 6.378(1), b ؍ 7.454(2), c ؍ 10.308(2) A s , ؍ 75.26(3), ؍ 79.37(3), ؍ 88.16(3)°. The structure can be represented with two types of layers: (i) hexagonal nets of Pb and (ii) similar layers but with 1/2 of Pb replaced by equal numbers of S and 2H. These layers are stacked in the sequence . . . BAABAA . . . Structural units can be identified in tetragonal and orthorhombic PbO which are also present in tribasic lead sulfate and provide a common basis for relating these structures. These units are defined by Pb-O bonds which are less than 2.5 A s which represent the strongest linkages. Two OH groups form weak hydrogen bonds to a single oxygen of a sulfate group. Lead polyhedra show both a range in coordination and Pb-O distances consistent with oxygen forming irregular polyhedra within the more regular cation framework. An alternative formula, 4PbO • H 2 SO 4 , may better represent the structure with the suggestion that H 2 and SO 4 groups replace Pb rather than the incorporation of H 2 O groups.