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A series of replicate flow-through cell experiments was conducted to characterize Cr isotope fractionation during Cr(VI) treatment by granular zero-valent iron (ZVI). Synthetic groundwater containing 50 mg L-1 Cr(VI) was pumped upward through a custom-made cell packed with ZVI under anaerobic conditions. The geochemical evolution of the system was monitored using pH and redox measurements, while aqueous effluent samples were retained for analysis of cations and Cr isotopes. Real-time, in situ X-ray absorption near edge structure (XANES) spectroscopy collected via a Kapton (R) window in the cell provided additional information on the speciation of the reaction products. Increases in d 53 Cr values corresponding to decreases in Cr(VI) concentration suggested the occurrence of redox processes. Spectroscopic results correlated well with the isotope data, indicating reduction of Cr(VI) to Cr(III). The isotope data did not appear to follow a single trend. A two-stage system was proposed to explain the complex isotope trend, where the rapid Cr removal was associated with very little fractionation (epsilon = -0.2 parts per thousand), whereas slower removal was associated with a greater degree of fractionation (epsilon = -1.2 parts per thousand to -1.5 parts per thousand). Reactive transport modeling was used to quantify distinct isotope fractionation values (epsilon), differentiated by a significant change in the Cr removal rate. (C) 2014 Elsevier Ltd. All rights reserved.