The total chromium concentrations by ICP-MS and HPLC-ICP-MS and the redox chromium species have been determined in rivers impacted by untreated wastes from tanneries at Fès (Morocco). The results obtained by two ICP-MS analysis methods showed significantly different chromium values at m/z 53. The higher values obtained with extemal calibration, can be attributed to matrix effects, especially ³⁷Cl ¹⁶O⁺ at m/z 53. This is confrmed on the chromatograms by the presence of a peak at 100s with the anomalous ⁵²Cr/⁵³Cr isotopic ratio. The hexavalent chromium was not detected. Two trivalent chromium monomer species, Cr(OH)(H₂0)₅²⁺ and Cr(H₂0)₆³⁺, were present in low concentrations. We suppose that the major part of chromium occurred as Cr(III) polymeric species which were not retained on the column. These Cr(III) forms are usually complexed with Cl⁻ or/and SO₄²⁻, used as tanning agents.