Methane, carbon dioxide, and hydrogen sulfide emissions from the hydroelectric dam of Petit Saut on the Sinnamary River in French Guyana have been measured over a 2 year period. Since the beginning of the reservoir filling (January 1994), 300 km2 of tropical forest have been submerged. Emissions of CH4 by diffusion and by bubbling into the atmosphere or by degassing of the water released into the river, as well as the stock of dissolved gases in the lake, and their temporal evolutions were determined. Maximum emissions of 800 t CH4 per day were reached in February 1995, corresponding to dissolved CH4 concentrations of 14 mg4L−1 in the water column. The biological oxidation of methane results in a strong oxygen consumption in lake and river waters. Total emissions of CH4 and CO2 from January 1994 to December 1995 were calculated from the whole data set, which also allows us to calculate the total carbon loss since reservoir filling. About 10% of the carbon stored in soil and vegetation was released in gaseous form within 2 years.