Chromium contamination in the area of the Radomka river basin in Poland has lead to significant environmental problems. Therefore, chromium redox transformations were investigated in the aquatic environment under conditions as close to natural as possible ± at intermediate pH and in the presence of original bottom sediment and dissolved oxygen. Reduction of Cr(VI) in the Radomka river water occurs at a low rate (t1/2 = 19 h). The rate of Cr(VI) reduction increased in the river water spiked by HA (t1/2 = 0.5 h), and Fe(II) (t1/2 = 2 min). The kinetics of Cr(VI) reduction by Fe(II) was dependent on the evolution of the forms of Cr(VI) and the competing Fe(II) oxidation reaction by dissolved oxygen. The reduction of Cr(VI) by humic acids appeared to be a significantly slower process than Fe(II) oxidation by O2. Chromium(III) present in the tanning liquor used and the aged solution of chromium sulphate was less prone to oxidation by MnO2 than Cr(III) introduced with chromium nitrate and sulphate solutions (7% and 10% conversion over 1 h in comparison with 55% and 32%, respectively). Chromium deposits in bottom sediment turned out to be resistant to oxidation by MnO2.