Porphyry-type molybdenum deposits, many of which are in China, supply most of the World’s molybdenum. Of particular importance are the molybdenum deposits located in the Qinling-Dabie region that are responsible for more than half of China’s molybdenum production. A feature that distinguishes this suite of deposits from the better-known Climax and Endako sub-types of porphyry molybdenum deposits is their formation from CO₂-rich magmatic-hydrothermal fluids. The role of CO₂, if any, in the transport of molybdenum by these fluids, however, is poorly understood. We conducted experiments on the partitioning of molybdenum between H₂O-CO₂, H₂O-NaCl, and H₂O-NaCl-CO₂ fluids and a felsic melt at 850 °C and 100 and 200 MPa. Here we show that the exsolution of separate (immiscible) brine and vapor leads to the very high brine D_Mo values needed for efficient extraction of Mo from the magmas forming Dabie-type porphyry molybdenum deposits.