The study presents δ⁶⁵Cu and δ⁹⁷Mo isotope values from cogenetic chalcopyrite and molybdenite found in veins and breccias of the Dahutang W‐Cu‐Mo ore field in China. The samples span a 3–4 km range. Both isotopes show a significant degree of fractionation. Cu isotope values in the chalcopyrite range from −0.31‰ to +1.48‰, and Mo isotope values in the molybdenite range from −0.03‰ to +1.06‰. For the cogenetic sulfide veined samples, a negative slope relationship exists between δ⁶⁵Cu and δ⁹⁷Mo values, which suggest a similar fluid history. Rayleigh distillation models the vein samples' change in isotope values. The breccia samples do not fall on the trend, thus indicating a different source mineralization event. Measured fluid inclusion and δD and δ¹⁸O data from cogenetic quartz indicate changes in temperature, and mixing of fluids do not appear to cause the isotopic shifts measure. Related equilibrium processes associated with the partitioning of metal between the vapor‐fluid in the hydrothermal system could be the probable cause for the relationship seen between the two isotope systems.