Contamination of the traditional gold-mining areas of Transbaikalia (Russia, Siberia) is described. To estimate the placer gold-mining impact on the ecological quality of river waters, extensive investigations were carried out in two river basins of the Lake Baikal catchment. Significant deterioration of river water quality (pH, turbidity, electric conductivity, and content of metals) was detected in the gold-mining sites. The extent of mercury contamination depended on the duration and intensity of mining activities. The most severe cases of mercury contamination were observed in areas with direct use of the mercury-amalgamation technique. Mercury concentrations at the gold-mining areas and at background sites varied from <0.001 to 0.78 mg kg−1 for bedrock and minerals, <1–183 ng m−3for the atmosphere, 0.013–3.59 mg kg−1 for soils, <5–5000 ng l−1, and <5–27 800 ng l−1 for dissolved and particulate water fractions, respectively, and 0.008–54.2 mg kg−1 for river-bottom sediments.