Various oxidation states of polyaniline, as film and powder, were used for reduction of highly toxic Cr(VI) to less toxic Cr(III) ion. The effects of various parameters such as synthesis method, physical and oxidation state of polyaniline, film thickness, solution pH and initial Cr(VI) concentration on the kinetics and efficiency of reduction. process were investigated. Results showed that a very broad concentration range of Cr(VI) solutions (10-10(5) ppb by emeraldine base and 10-10(6.5) ppb by leucoemeraldine) can be efficiently (>98%) reduced by polyaniline as film or powder. Cr(VI) solutions with concentrations higher than these ranges were caused to the overoxidation and degradation of polymer. Decreasing of solution pH, increased the kinetics and performance of reduction process, but lowered the Cr(VI) concentration ranges caused the overoxidation and degradation of the polymer. Higher reduction efficiencies were obtained for thicker electrochemically synthesized polyaniline films and for thinner chemically prepared polyaniline free standing films. The ability or capacity of various forms of polyaniline in Cr(VI) reduction was evaluated as the milligrams of Cr(VI) reduced by one gram of various forms of polyaniline. (c) 2007 Elsevier B.V. All rights reserved.