THE atmospheric budget of carbon compounds can be balanced only by invoking a significant ’missing sink’ for carbon dioxide(1-3). Identifying this sink requires a knowledge of CO2 fluxes at global and local scales. The former can be estimated from global averages of CO2 concentration and isotope composition(4-9); local-scale measurements have been made by analysing individual eddies of air(10,11). In both cases, the net CO2 exchange is the sum of two opposing fluxes : uptake by gross primary productivity and release by respiration. Here we show that these two components can be estimated separately at the local scale from small vertical gradients in C-13 and O-18 in atmospheric CO2 above vegetation. By also analysing the O-18 content of moisture in the air samples, we can estimate evapotranspiration rates, providing information on water exchange between the biosphere and atmosphere(12). We suggest that this approach can be extended to the regional scale.