Porphyry Cu-Mo deposits and porphyry Mo-Cu deposits are the most important sources of world’s Mo resources, each providing comparable Mo reserves. Although sharing the same characteristics in fluid source and evolution pattern and Cu-Mo correlation, those two types of deposits demonstrate many differences in Cu/Mo ratio, ore grade, mineral association, hydrothermal alteration type, and especially in primitive fluid composition, with fluids from porphyry Cu-Mo deposits containing more sulfur and showing higher ratios of Cl-/F-, SO₂/H₂S, as well as H+/ K+ than those from porphyry Mo-Cu deposits .Two factors may influence the final Cu/Mo ratio in a porphyry deposit during fluid evolution: 1) different behaviors of Cu and Mo in hydrothermal fluids (e.g., Cu is usually transported as chlorine or sulfur complexes in fluids, the stabilities of which are dominantly controlled by temperature, while Mo mainly appears as hydroxyl or chlorine complexes, and their precipitation are principally influenced by pressure shift); and 2) change of f(O₂), f(S₂), pH and evolution path of the primitive fluids.