The oxidation of trivalent chromium, Cr(III), to the more toxic Cr(VI) in chlorinated water is thermodynamically feasible and was the subject of the study. The study found that free available chlorine (FAC) readily converts Cr(III) to Cr(VI) at a rate that is highly dependent upon pH, total organic carbon (TOC), and chloride concentrations while combined chlorine (CAC) does not oxidize Cr(III). The highest oxidation rate occurs in the 5.5-6.0 pH range. The aquatic humus in natural water, however, inhibits while chloride concentration catalyzes the rate of Cr(VI) formation. As expected, the initial Cr(III) oxidation rate increased with increasing FAC concentration and Cr(III) level. Monochloramine, a form of CAC, did not oxidize Cr(III) at any tested pH between 6 and 8.5. The results suggest that the oxidation of Cr(III) and Cr(VI) would rarely occur to a significant extent during drinking water chlorination because of the presence of naturally occurring organics (TOC), the low concentrations of Cr(III) in natural waters, and the probable removal of insoluble Cr(OH)3 during coagulation. However, in CR(III)-contaminated waters that are relatively free of organic contamination and have pH's in the 5-7 range, FAC readily converts Cr(III) to the more toxic hexavalent variety.