Abstract. [C(NH2) 314[P408S4] , Mr = 620.46, monoclinic, P2Jn, b=13.126(8), c = 8.919 (6) A,/3 = 105.74 (5) °, V = 1227 (3) A 3, Z = 2, Dm= 1"64, Dx = 1"678 Mg m-3, A(Mo Ka) = 0"7107A, /z =0.693mm-~, F(000)=640, T= 293 K, final R = 0.027 for 2356 independent reflections. The P408S4 ring anions are centrosymmetric. Among the four external S atoms two belonging to two successive P atoms are located above the ring plane while the two remaining ones are below. The two independent P atoms of the ring have a highly distorted tetrahedral coordination involving three O atoms and one S atom. The guanidinium cations, forming planar groups, are connected to the phosphoric ring anions by weak hydrogen bonds.