A mathematical model for leaf photosynthesis is constructed. The model relates the net rate of carbon dioxide exchange per unit leaf area to the ambient carbon dioxide and oxygen concentrations and the light-flux density incident on the leaf surface. The behaviour of the model under steady-state conditions is examined and discussed. Simulated carbon dioxide and light-response curves for the net carbon dioxide exchange rate accord well with experimental observation. The model describes an inhibition of the net exchange rate by increasing oxygen concentrations which is similar to the commonly observed inhibition.