beta -La(NH4)(SO4)(2) crystallises with the monoclinic symmetry [a = 6.9365(4) Angstrom, b = 9.0055(5) Angstrom, c = 5.4541(4) Angstrom, beta = 90.672(8)degrees and V = 340.68(3) Angstrom (3), S.G. Pn, Z = 2]. Its structure has been solved ab initio from X-ray powder diffraction data and refined by the Rietveld method (RF = 0.06 and R-wp = 0.10). The lanthanum atoms are nine-fold coordinated to sulfate oxygen atoms according to a monodentate-bidentate mode. Through bridging sulfate groups, the LaO9 polyhedra are joined into an anionic layer-like structure parallel to (010). To stabilise the structure the NH4+ cations lying between the layers form short contacts with the terminal O atoms in the sulfate groups, The thermal behaviour of beta -La(NH4)(SO4)(2) and alpha -LaK(SO4)(2), investigated by thermodiffractometry, is also described. The ammonium beta phase decomposes into the oxysulfate La2O2SO4 through the departure of ammonium sulfate in the first stage. According to a similar thermal treatment, the potassium a phase transforms into a polymorph (beta) and the phase transition process is reversible upon cooling, The diffraction patterns of these new varieties of LaK(SO4)(2) have been indexed.