A batch recycling fluidized bed electrochemical reactor system was used to recover Cr(VI) from a very dilute solution containing 2.5 g Cr/L at pH 2. It was found that the electrowinning rate is controlled by mass transfer and that the reduction process of Cr(VI) is interrupted by the formation of Cr(H2O)(6)(3+). The Cr2O72- and Cr(H2O)(6)(3+) concentration-time relationship can be predicted by a plug flow model. A finite difference method and a complex optimization technique were used to estimate the mass transfer coefficient. At Re-p between 10 and 30, the Cr2O72- and Cr(H2O)(6)(3+) mass transfer coefficients lie between 1.52-3.20 x 10(-6) and 1.17-2.50 x 10(-6) cm/s, respectively, and increase with the Reynolds number based on the particulate cathode.