The stability relations between scheelite and tungstenite and between powellite and molybdenite were experimentally determined on the basis of the following reactions:CaWO 4 + SiO 2 + S 2 = CaSiO 3 + WS 2 + 3/2 O 2 CaMoO 4 + SiO 2 + S 2 = CaSi 3 + MoS 2 3/2 O 2 Under hydrothermal conditions, the oxygen and sulfur fugacities for the above reactions were controlled simultaneously by a single or combination of solid buffer assemblages. At P f = 1,000 bars and T = 577 degrees C, the equilibrium curve for reaction (1) passes through these points on the log f (sub S 2 ) - log f (sub O 2 ) diagram: (-3.0, -21.7), (--7.1, -24.4), and (-12.9, --28.3), whereas the equilibrium curve for reaction (2) passes through these points on the same diagram: (-3.0, -16.3), (-7.1, -19.0), and (-12.9, -22.9).With a proper supply of Ca, Mo, and W, the formation of powellite requires over five orders of magnitude of f (sub O 2 ) higher than the formation of scheelite unler the same f (sub S 2 ) conditions. The restriction of f (sub S 2 ) imposed upon the formation of tungstenite as compared with molybdenite is even more rigorous under the same f (sub O 2 ) conditions, i.e., over eight orders of magnitude difference. The common natural association of scheelite with molybdenite is consistent with their wide overlap of log f (sub S 2 ) -- log- f (sub O 2 ) stability fields. It is also concluded that powellite cannot occur with tungstenite and the formation of either of the two minerals alone requires an unusual geological environment, i.e., very high f (sub O 2 ) /low f (sub S 2 ) for the former and very low f (sub O 2 ) /high f (sub S 2 ) for the latter.