In this study, the ability of mackinawite (FeS) to reduce Cr(VI) was investigated as a function of the initial aqueous Cr(VI) concentration (1 × 10−4–5 × 10−3M) at pH 5 and fixed ionic strength under a nitrogen atmosphere. X-ray photoelectron spectroscopy was used to characterize the surface after reaction and to identify the oxidation products. Aqueous Cr(VI) concentration removed from solution was determined using the diphenylcarbazide colorimetric method. Mackinawite was found to act as an efficient Cr(VI) removal material. Surface analysis revealed that all detectable chromium occurred as Cr(III), even at the highest initial Cr(VI) concentration. Strong attenuation of the Fe 2p and S 2p signals from mackinawite counterbalanced the increase of the Cr 2p and O 1s levels when the initial Cr(VI) concentration increased. Clear evidence for the oxidation of Fe(II) to Fe(III), as well as hydroxylation of the surface, was provided by the Fe 2p and O 1s levels, respectively. Furthermore, the S 2p level indicated that S(-II) was the main component, whatever the initial Cr(VI) concentration, with small amounts of polysulphides and elemental sulphur at the surface. These observations at pH 5 show that the mechanism of removal of Cr(VI) is through the reduction of Cr(VI) to Cr(III) and results in the formation of oxidation products such as Cr(OH)3 and/or FexCr(1−x)(OH)3 precipitates at the reacted mackinawite surface. Copyright © 2002 John Wiley & Sons, Ltd.