We report structure analysis of a new family of rare-earth hydroxides Ln(2)(OH)(4)SO4 center dot 2H(2)O (Ln = Pr, Nd, Sm, Eu, Gd, Tb) from synchrotron X-ray and electron diffraction data. Rietveld profile analysis revealed that all members were isostructural and crystallized in a face-centered monoclinic system A2/m (No. 12), in which the monoclinic angles were approximately equal to the right angle, varying from 90.387(1)degrees for Pr sample to 90.0718(3)degrees for Tb sample. The structure consisted of LnO(9) polyhedra connected by mu(3)-hydroxyl groups and mu(2)-water molecules, forming a corrugated two-dimensional layer, which was pillared by bidentated sulfate ions. This series of compounds had a supercell a' = 2a, b' = 2b because of the local orientation ordering of SO42-. Structural features along the series, such as unit-cell parameters and average Ln-O distances, represented a progressive contraction associated with the shrinking radius of the lanthanide cations from Pr to Tb.