A one-dimensional mathematical model of gas-diffusion electrodes for oxygen reduction is presented to investigate the effect of gas composition and mass-transport limitation on the current-potential curve. A simple explicit analytical solution has been obtained, which works well for conditions relatively far from the limiting current situation. In the proximity of the limiting current this basic model has to be slightly modified and an explicit analytical solution cannot be found. Nevertheless, the form of the equations remains simple, making possible direct and easy identification of the parameters of the electrode. The proposed models have been compared favorably with experimental data and with the results of a more complex electrode model, developed by other authors. In addition, they have been used as a tool to estimate the gradual aging of an air electrode.