In this study, the long-term stability of Cr(VI) in the FeSO4 and H2SO4 (FeSO4-H2SO4) treated chromite ore processing residue (COPR) after 400 curing days and the stabilization mechanisms were investigated. FeSO4-H2SO4 treatment significantly reduced toxicity characteristic leaching procedure (TCLP) and synthetic precipitation leaching procedure (SPLP) Cr(VI) concentrations to lower than the regulatory limit of 1.5 mg L-1 (HJ/T 301-2007, China EPA) even for the samples curing 400 days, achieving an outstanding long-term stability. Our independent leaching tests revealed that H+ and SO42- have synergistic effect on promoting the release of Cr(VI), which would make Cr(VI) easier accessed by Fe(II) during stabilization. The contributions of H+ and SO42- to Cr(VI) release ratio were 25%-44% and 19%-38%, respectively, as 5 mol H2SO4 per kg COPR was used. X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and alkaline digestion analyses were also employed to interpret the possible stabilization mechanism. Cr(VI) released from COPR solid was reduced to Cr(III) by Fe(II), and then formed stable FexCr(1-x)(OH)(3) precipitate. This study provides a facile and reliable scheme for COPR stabilization, and verifies the excellent long-term stability of the FeSO4-H2SO4 treated COPR. (C) 2016 Elsevier B.V. All rights reserved.