The impoundment of the Petit-Saut reservoir (French Guyana) in 1994 flooded over 300 km2 of tropical forest, causing the anoxia of the hypolimnion and a considerable emission of GHGs. The parallel evolution of dissolved iron and methane suggested that both originated from the reduction of flooded soils. The comparison of flooded with control soils showed a 50% loss in organic matter, as well as a drop in its hydrogen index, and an increase in Δ13C. The strong correlation between organic matter and iron oxide content in control soils became weak in flooded soils, while the weak correlation between Fe-oxide and amorphous-Fe in control soils became strong in flooded soils, suggesting a massive transformation of Fe-oxide into amorphous Fe. Measured in the lab in anoxic conditions, the carbon mineralization rate of flooded and control soils was proportional to their carbon content, suggesting continuous mineralization of organic matter in flooded soils. In contrast, the rate of methane production by flooded soils was very low. In addition, the 14C ages of flooded soils, methane bubbles in flooded soils, and dissolved methane in the hypolimnion were close to the age of the reservoir. This suggests that a fraction of reactive soil OM, bound to iron oxyhydroxides and released by iron reduction and dissolution when soils are flooded, is the source of the methane emitted by tropical reservoirs.