The solubility of MoO[subscript]2 and MoS[subscript]2 have been measured in NaCl solutions at temperatures between 300 and 450°C under water-vapor saturation pressures. Experiments were performed in Barnes type vessels of capacity of 1.1 and 0.5 liters. Oxygen and sulfur fugacities were controlled by the buffer assemblages Fe[subscript]3O[subscript]4-Fe[subscript]2O[subscript]3 (MH), Fe[subscript]3O[subscript]4-Fe[subscript]2O[subscript]3-FeS[subscript]2 (MHP), and Fe[subscript]3O[subscript]4-FeS[subscript]2-FeS (MPP);In the sulfur-free system (buffered by MH), the total concentration of molybdenum in solutions varies from several tens of ppm to over 3000 ppm, increasing with NaCl concentration in the fluids. In sulfur-rich systems (buffered by MHP or MPP), however, the solubility is decreased by 2 to 3 orders of magnitude, with most of them ranging from 0.1 to 10 ppm. At all temperatures, molybdenite is more soluble in MHP-buffered solutions than it is in MPP-buffered solutions, indicating an effect of f[subscript] O2 and f[subscript] S2 on the solubility;A speciation scheme was also devised for aqueous molybdenum complexes based on the solubility data of MoO[subscript]2. At lower temperatures, or low Na[superscript]+ concentration, HMoO[subscript]4[superscript]- is the dominant Mo (6+) species. With increasing temperature, however, ion pairs of NaHMoO[subscript]4° and Na[subscript]2MoO[subscript]4° predominate in the fluids. Molybdenum species with lower oxidation state (5+) in the forms of MoO(OH)Cl[subscript]2 and MoO(OH)[subscript]2Cl are stable in solutions of relatively low f[subscript] O2 and pH values. These species, coupled with their thermodynamic properties, are used to predict the solubility of molybdenite. Both calculated and measured solubility agrees very well, suggesting that the dissolution or precipitation of MoS[subscript]2 are governed by reactions like MoS[subscript]2+ 1/4O[subscript]2+ 5/2H[subscript]2O+ H[superscript]+ + Cl[superscript]- = MoO(OH)[subscript]2Cl + 2H[subscript]2S & MoS[subscript]2+ 1/2O[subscript]2+ 3H[subscript]2O+ nNa[superscript]+ = Na[subscript]nH[subscript]2-nMoO[subscript]4°+ nH[superscript]+ + 2H[subscript]2S at temperatures above 300°C.