Résumé : In the frame of the BIOGEST project, the full transient, one-dimensional, reactive-transport model CONTRASTE has been extended for the computation of biogases in the Scheldt estuary. The CONTRASTE model (Coupled, Networked, Transport-Reaction Algorithm for Strong Tidal Estuaries) provides a satisfactory description of the estuarine residual circulation (including daily freshwater discharge and a complete description of the tide) and a flexible implementation of the various physico-chemical and biological transformations, including both kinetically-controlled and equilibrium reactions. The model allows resolution of the complex, nonlinear collective behaviour of this type of system and investigation of the non-steady-state phenomena which govern estuarine dynamics. Variables currently implemented in the model include salinity, suspended matter, oxygen, inorganic carbon species, degradable organic carbon and nitrogen, inorganic nitrogen species, freshwater and marine phytoplankton. Biological processes described are heterotrophic respiration, primary production, nitrification and denitrification. Equilibrium formulations allow for DIC and NH4+/NH3 speciation. Physical processes include gas transfer at the water/air interface, depending on both wind speed and current velocity. pH profiles are explicitly computed and constitute a very sensitive check of the overall model consistency. Results of the CONTRASTE model are in very good agreement with the measured longitudinal distribution of the variables considered, in particular O2, pH, pCO2 and N2O concentrations. However, discrepancies are observed between the calculated fluxes of CO2 and those estimated using an in situ floating chamber. It is shown that the evaluation of gas transfer can be affected by serious errors if the variations due to changes in current velocity and water depth during one tidal cycle are not taken into consideration. The model also shows that the fluxes of biogases in estuaries are greatly influenced by the quasi-exponential increase of the exchange surface area with decreasing distance to the sea. Our estimation of the total daily flux of O2, CO2 and N2O is equal to +28500, -19000 and -17 kmoles.day-1 respectively for the Scheldt estuary in July 1996.