Many developing countries have regions of high demographic density, where untreated residuary waters from different sources are often discharged into rivers, streams and other water bodies. This paper discusses the reducing action of organic matter of anthropic origin on the mercury redox cycle in the Jundiaí River impacted by discharged wastes, and on the Piraí River, a non-impacted water body. The total mercury concentrations in these locations vary from 1.7 to 32 ng L -1 in the former and from 0.6 to 10.6 ng L -1 in the latter. Dissolved organic carbon concentrations of up to 68.3 and 6.5 mg L -1 were observed, confirming the higher impact on the Jundiaí River. It was found that an inverse correlation between the concentration of dissolved organic carbon and total mercury was stronger in the Jundiaí River, given that it receives higher organic loads, suggesting that organic matter exerts a reducing action on mercury, which is released as gas into the atmosphere. 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