The kinetics and stoichiometry of the reduction of hexavalent chromium (Cr(VI)) with ferrous iron (Fe(II)) were examined in systems with and without aquifer solids. Cr(VI) reduction was rapid in the absence of solids, but demonstrated slower and more complex kinetics in the presence of aquifer solids. The aquifer solids removed Fe(II) from solution and a portion of the reducing capacity of Fe(II) was transferred to the aquifer solids. The solid phases were then able to continue to remove Cr(VI). This suggests in situ treatment of Cr(VI) by Fe(II) injection would be feasible in the aquifer environment. In general, re-oxidation of reduced chromium by molecular oxygen was not observed in our systems over time periods of nearly I year, suggesting that the potential for chromium solubilization under these oxidizing conditions will be low. An empirical model was developed to describe the reduction kinetics of Cr(VI) in the presence of solids. The model assumes that the reaction is brought about by pseudo-species of iron that react instantaneously, rapidly and slowly with Cr(VI). A fourth pseudo-species is assumed to be non-reactive. Model coefficients were determined by non-linear regression. The model was able to describe observed concentrations of chromium well, but analysis of model errors indicated the potential existence of a distribution of species with different reaction rates rather than just three distinct species. Another model was developed to predict concentrations of different pseudo-species depending on the total amount of Fe(II) added and the amount of aquifer solids present. This model assumed that pseudo-species could result from intrinsic characteristics of the aquifer solids as well as being formed by addition of Fe(II), which could sorb to the aquifer solids. (C) 2002 Elsevier Science B.V. All rights reserved.