Numerical simulations are carried out in this paper, to study the effect of air stoichiometry change on a single-channel proton exchange membrane (PEM) fuel cell undergoing load change. For this purpose, the air stoichiometry change rate and pattern, as well as its lagging time to the load change and initial value, are taken into account in this model, respectively. The transient response of cell potential to load change has been investigated and the undershoot behavior has been observed. Detailed results are further presented to show the transient response of the cell in terms of local current density and oxygen mass fraction distributions. This work shows that the extent of air dilution/starvation down the channel depends on the air stoichiomtric flow ratio change rate, pattern, etc. It is also shown this undershoot can be eliminated by adjusting the initial air flow ratio, or by improving the air flow ratio change rate, etc. Finally, the validity of the numerical model in this paper is verified by the comparison between the model results and experimental data, showing that this numerical model can be resorted to reveal the effects of air stoichiometry change on the dynamic behavior of a PEMFC. (c) 2008 Elsevier B.V. All rights reserved.