Micrometeorological measurements of the surface flux of carbon dioxide were made at a number of spatially separate sites within the HAPEX-Sahel experimental area. Differences in the timing of plant development caused by differences in rainfall (both quantity and frequency) over the experimental area exert a major influence on the absolute magnitude of the surface flux of carbon made simultaneously over the experimental area. Surface fluxes are presented for the three major types of vegetation in the Sahelian landscape, namely, millet, savannah and tiger bush. The atmospheric and surface controls on the surface flux of CO2 are described for these three land surfaces and the fluxes are compared between sites. The comparison shows that modellers need to take into account the heterogeneity not only of the type of surface vegetation but also the variability in the life cycle stage of the vegetation, when scaling up to larger areas.