Mercury pollution is a preoccupying environmental problem affecting French Guiana ecosystems. Mercury (Hg) concentrations found in fei-rallitic soils of the dense Guianese forest are high and variable. Natural Hg geochemical background is the result of a long telm accumulation from two sources in the soil, in situ, weathering ofparental material and atmospheric inputs from oceans principally. Abnormally high levels of mercury are related to gold-mining activities. To evaluate the proportion of mercury from weathering processes and from atmospheric inputs, we used a mass balance calculation based on trace elements. This approach reveals that even in areas considered to be natural environments, the proportion of Hg derived from weathering appears to be negligible compared to the atmospheric origin. Soils located close to gold-mining sites are strongly affected by mercury pollution. Pellehatioll and accumulation of atmospheric inputs as well as Hg redistribution in soil profiles depend on the hydrodynamical propeliies, redox conditions and the quality and quantity of carrier phases. Key-words: Mercury, soils, atmospheric inputs, geochemical background, pollution, gold-mining.