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Carcinogenic effects of hexavalent chromium in waters are of concern in many countries worldwide. We explored Cr isotope systematics at 11 sites in the Czech Republic and Poland. Geogenic Cr pollution was associated with serpentinite bodies at former convergent plate margins, while anthropogenic Cr pollution resulted from electroplating, tanning, and the chemical industry. Cr(VI) concentration in geogenic waters was less than 40 ppb. Anthropogenic waters contained up to 127,000 ppb Cr(VI). At both geogenic and anthropogenic sites, where known, the source of pollution had a low delta Cr-53 (<1 parts per thousand). delta Cr-53 of geogenic and anthropogenic waters was up to 3.9 and 5.8 parts per thousand, respectively. At both serpentinite-dominated and industrial sites, delta Cr-53(VI)(aq) was shifted toward higher values, compared to the pollution source. At the industrial sites, this positive delta Cr-53 shift was related to Cr(VI) reduction, a process known to fractionate Cr isotopes. At geogenic sites, the origin of high delta Cr-53(VI)(aq) is tentatively ascribed to preferential release of Cr-53 during oxidation of soil Cr(III) and its mobilization to water. delta Cr-53(VI) of industrially contaminated waters was significantly higher (p < 0.001) compared to delta Cr-53 of waters carrying geogenic Cr(VI), implying that either the effective fractionation factor or process extent was greater for Cr(VI) reduction than for Cr(III) oxidation.