The structure of lanthanum oxysulfate (LaO)(2)SO4 was studied with neutron, electron, and X-ray diffraction using the Rietveld refinement method. The unit cell is determined to be monoclinic (a = 14.349(1), b = 4.2852(5), c = 8.386(1) Angstrom, beta = 107.00(1), Sp.gr. C2/c). The structure is similar to that of grandreefite and consists of the alternative sulfate and (LaO) layers. Every sulfate oxygen is coordinated with two lanthanum atoms. The experimental data were confirmed by the energy minimization calculations.