Copper isotope fractionation factors have been determined experimentally at temperatures of 250 and 300°C for fluid salinities of 1, 3, 5 and 7 molal Cl-, and starting acidity of 0, 0.01, 0.1, 0.2 and 0.6 molar HCl. The experiments, which follow a partial dissolution of natural chalcopyrite in a hydrothermal fluid from 7 to 30days, yield a significant range of leached copper-residual chalcopyrite isotopic fractionations. These appear to be pH, temperature and salinity dependent, with pH showing the greatest apparent control over the magnitude of fractionation. For weakly acidic conditions, preferential incorporation of the light isotope into the leached phase(s) generally occurred. Experiments gained a maximum fractionation magnitude at intermediate pH (4-7), where the leached copper-chalcopyrite fractionation magnitude reached to -1‰, depending on fluid salinity and temperature. Fractionation decreased at higher and lower solubility (pH) conditions. The highest pH experiments showed a small positive fractionation (approx. +0.2 to 0.3‰). Limited reproducibility of the experiments is related to unconstrained physicochemical conditions (primarily pH and fO2). A major control on the fractionation magnitude appears to be the degree of partitioning of Cu between vapor and liquid phases, which is controlled by pH and salinity. The copper-bearing vapor species permitting significant mass transfer to the vapor apparently differ with salinity and temperature. Under certain experimental conditions one or more vapor species forms and produces significant isotopic fractionation between the copper in the liquid and copper in the vapor. The experiments indicate that the likelihood of observing significant isotopic fractionation in hypogene ore forming systems depends on the pH of the mineralizing fluid and the controls over partitioning of Cu between a vapor and liquid phase.