Three new lanthanide metal polysulfides have been synthesized in supercritical ammonia : [Yb(NH3)(8)][Cu(S-4)(2)]. NH3 (I), [Yb(NH3)(8)][Ag(S-4)(2)]. 2NH(3) (II), [La(NH3)](9)[CU(S-4)2] (III). Compounds were prepared in sealed quartz tubes which were filled to one-third volume with liquid ammonia and heated at 170 degrees C for 3 days. Compounds I and II were obtained from the reaction of a premade ytterbium polysulfide with M and S in the ratio of 1:1:4, while [La(NH3)(9)][Cu(S-4)(2)], III, was Obtained by reacting La, Cu, and S in the ratio of 1:1:8. In all these compounds, the lanthanide cation is coordinated only by ammonia, while the M(+) is chelated by S-4(2-) to form metal tetrasulfide rings. [Yb(NH3)(8)][Cu(S-4)(2)]. NH3 and [Yb(NH3)(8)][Ag(S-4)(2)]. 2NH(3) contain [M(S-4)(2)](3-) anionic units with M(+) in a distorted tetrahedral coordination environment chelated by two S-4(2-) groups. Compound III, [La(NH3)(9)][Cu(S-4)(2)], also contains anionic [Cu(S-4)(2)](3-) units; however, Cu+ is in a trigonal planar coordination geometry due;to the presence of one monodentate S-4(2-). All the compounds are unstable with respect to loss of ammonia, and decomposition yielded known ternary and binary phases. Crystallographic data for the compounds are the following : I, space group = , Z = 2, a = 10.024(4) Angstrom, c = 9.753,(4) Angstrom; II, space group = P2(1)2(1)2(1), Z = 4, a = 11.177(4) Angstrom, b = 20.472(6) Angstrom, c = 9.754(3) Angstrom; III, space group = P2(1)/n, Z = 4, a = 9.891(2) Angstrom, b = 12.600(3) Angstrom, c = 17.812(3) Angstrom, beta = 103.57(2)degrees.