The objective of this work is to quantify long-term emissions of two major greenhouse gases, CO2 and CH4, produced by the decomposition of the flooded organic matter in tropical artificial reservoirs. In a previous paper [Galy-Lacaux et al., 1997], gas emissions from the tropical reservoir of Petit Saut (French Guiana) were quantified over the first two years after impounding. This work presents emission fluxes and distributions of dissolved methane and carbon dioxide measured in the reservoir of Petit Saut over three and a half years, since the beginning of impounding (1994) and during operation (1995--1997). To assess long term emissions, an experimental campaign was conducted on four hydroelectric reservoirs (Taabo, Buyo, and Ayame I and II) built between 1960 and 1980 in the Ivory Coast. Average dissolved CH4 concentration in the water column of the Petit Saut reservoir first increased, up to a maximum of 14 mgL-1, in May 1995. Then the time course of dissolved CH4 over the three and a half year period, showed periodical variations. These changes were related to changes in the inlet water flow and the residence time of water in the reservoir. In the older African reservoirs, average dissolved methane concentrations were lower and ranged between 0.20 and 0.32 mgL-1. The whole data set allows us to propose an analytical algorithm in order to predict the time course of dissolved CH4 concentration in the Petit Saut reservoir. Temporal variations of total CH4 and CO2 emissions from the reservoir over three and a half years were extrapolated with this algorithm to calculate long term carbon losses. Over a 20-year period the estimated carbon losses in the form of CO2 and CH4 were dominated by the outlet fluxes of dissolved gases (2160¿400Gg(C)), and they correspond to a total net carbon loss of 3.2 Tg(C). The contribution of the Petit Saut reservoir to greenhouse gas emission, over 20 years, is estimated to be 66¿20Tg of CO2 equivalent (56 Tg as CH4 and 9.7 Tg as CO2). ¿ 1999 American Geophysical Union