<p>This study reports Mo isotope data for molybdenite and whole-rock samples from the giant Leimengou porphyry Mo deposit in the Qinling Orogen (China), and interprets the Mo isotope fractionation as being due to magmatic-hydrothermal processes. The average δ^98/95Mo values of syn-ore porphyritic monzogranite (PMG) and porphyritic K-feldspar granite (PKG), post-ore quartz monzonite porphyry (QMP), and Taihua Supergroup gneiss are + 0.29 ‰, − 0.26 ‰, 0 ‰, and + 0.14 ‰, respectively. The values for molybdenite decrease from + 0.19 ± 0.17 (‰) in Stage II quartz-molybdenite veins to − 0.53 ± 0.22 (‰) in Stage III quartz-pyrite-molybdenite veins. The εNd(t) values of the PMG and PKG are negative, consistent with a crust-dominated source for these intrusions; the PKG is an evolved peraluminous granite interpreted to have formed by fractional crystallization of the magma that generated the PMG. In addition, the more evolved PKG (high SiO₂ content and Mo/Ce ratio) has a negative δ^98/95Mo value, whereas the PMG from which it was derived (less fractionated with a lower SiO₂ content and Mo/Ce ratio) has a positive δ^98/95Mo value. The lower δ^98/95Mo value of the PKG is interpreted to reflect fractional crystallization of plagioclase. Significantly samples of PMG and PKG with high Mo contents and Mo/Ce ratios have low δ^98/95Mo values, whereas those with low Mo contents and Mo/Ce ratios have high δ^98/95Mo values. The samples with low Mo contents represent magma that lost Mo to magmatic fluid, and therefore their high δ^98/95Mo values are interpreted to indicate that the light Mo isotope preferred the fluid. This unusual Mo isotopic behavior is likely universal for collision-type porphyry Mo deposits but is different from that of Climax-type porphyry Mo deposits and may reflect the high content of CO₂ in collision-type fluids and its low content in Climax-type fluids.</p>