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Natural attenuation of groundwater contaminated by toxic hexavalent chromium [Cr(VI)] was studied at two industrial sites in the eastern U.S. Concentration and isotope composition of the residual Cr(VI) were determined at site A in Connecticut whose aquifer was polluted by electroplating solutions between 1930 and 1995, and at site B in New Jersey whose aquifer was polluted by soluble chromium-processing wastes between 1911 and 1963. Site A was characterized by lower mean Cr(VI) concentrations (2 mg L-1) and lower mean delta(CrCr)-Cr-53(VI) values (1.84%) in groundwater than site B (200 mg L-1 and 2.89%, respectively). delta(CrCr)-Cr-53(VI) values were strongly positively correlated with dissolved organic carbon (DOC; p = 0.002), but not with dissolved ferrous iron [Fe(II)], or divalent manganese [Mn(II)], indicating that DOC may have played a major role in spontaneous reduction of the mobile carcinogenic Cr(VI) to immobile non-toxic Cr(III). At site B, the supply of DOC may be nearly unlimited, due to the presence of a buried organic meadow mat at a depth of six meters below the ground surface. At site A, contaminated groundwater has been extracted and treated since 1995. The mean delta(CrCr)-Cr-53(VI) values of the residual aqueous contamination at site A significantly decreased over the past 15 years (p < 0.05), possibly suggesting re-oxidation and remobilization of previously precipitated pollutant Cr.