The structure of dipotassium mu(3)-oxido-tris[sulfitomercurate(II)], K(2)[O(HgSO(3))(3)], is characterized by segregation of the K(+) cations and complex [O(HgSO(3))(3)](2-) anions into layers parallel to (010). The anion has m symmetry and is a new example of a mu(3)-oxido-trimercurate complex with a central [OHg(3)] core. This unit adopts the shape of a flat, almost trigonal, pyramid (mean O-Hg = 2.072 A and mean Hg-O-Hg = 110.8 degrees). The two independent Hg-S bonds have nearly the same length (mean Hg-S = 2.335 A). Due to intermolecular O...Hg donor-acceptor interactions greater than 2.65 A, the O-Hg-S fragments are slightly bent. The [KO(9)] coordination polyhedron of the K(+) cation approaches a distorted tricapped trigonal prism with a [6+1+2] coordination.