The performance of an air breathing proton exchange membrane (PEM) fuel cell stack has been experimentally measured to investigate the steady-state and transient effects of temperature, humidity and air flowrate. The results show that hydrogen leaks to the cathode through the membrane causing internal heating of the fuel cell. The leakage rate is found to be linearly dependent on the pressure difference between the hydrogen side and air side which is at atmospheric pressure. Temperature was found to not have a significant effect on the PEM performance, except through its indirect effect on humidity. The humidity of the membrane is found to be the most significant variable in determining the fuel cell performance. The airflow also influences the performance of the fuel cell directly by supplying oxygen and indirectly by influencing the humidity of the membrane. Experiments show that an optimum air flowrate exists that is much larger than required for stoichiometric oxidation of the fuel.