The vaporization behavior of the Nd-S and Yb-S systems was studied at 1952-2229K and 1688-1842K respectively. The congruent vaporizing compounds found in both systems are nonstoichiometric compounds, i.e., NdS[subscript]1.38 and Yb[subscript]0.877S. Mass spectrometric investigation showed that the congruent vaporization reactions for NdS[subscript]1.38 are: (I) NdS[subscript]1.38(s) = NdS(g) + 0.38 S(g) and (II) NdS[subscript]1.38(s) = Nd(g) + 1.38 S(g); and the congruent vaporizing reaction for Yb[subscript]0.877S is: Yb[subscript]0.877S(s) = 0.877 Yb(s) + S(g). The equilibrium partial pressures of the gaseous species, the second- and third-law enthalpies and entropies of the vaporization reactions were obtained;Two new thulium sulfides: Tm[subscript]8S[subscript]11 and Tm[subscript]15S[subscript]22 were discovered by vaporization of Tm-S samples using the Knudsen effusion method. Single crystals of both compounds were obtained and their structures were solved by single crystal x-ray diffraction techniques. Tm[subscript]8S[subscript]11 crystallizes in an orthorhombic space group Cmcm with a = 3.7486(4), b = 12.6160(8), c = 34.932(3)A and it has four formula units per cell. The structure type of Tm[subscript]8S[subscript]11 is new and the stoichiometry has never been previously reported for rare earth chalcogenides. Tm[subscript]15S[subscript]22 is isostructural with Ce[subscript]4Lu[subscript]11S[subscript]22. The magnetic susceptibility of Tm[subscript]8S[subscript]11 indicated that all Tm in Tm[subscript]8S[subscript]11 have oxidation state +3 and that there are eight delocalized electrons per cell.