Currently there has been very little reliability or end-of-life analysis conducted for polymer electrolyte membrane fuel cell (PEM) stacks, and detailed designs of PEM systems are still in a rapid evolutionary stage. Voltage degradation as a fuel cell ages is a widely observed phenomenon and results in a significant reduction in the electrical power produced by the stack. Little systematic information has been reported, however, and this phenomenon has not been included in electrochemical models. An earlier work described the development of the generalised steady state electrochemical model (GSSEM) which accepts as input the values of the operating variables (anode and cathode feed gas pressure and compositions, cell temperature and current density), and cell design parameters such as the active area and Nafion membrane thickness. This work will introduce new terms to the model to account for membrane electrode assembly (MEA) ageing, which is a factor in the durability of the stack. One term is based on the concept that the water-carrying capacity (a principal factor in membrane resistance) of the membrane deteriorates with time-in-service. A second term involves the apparent catalytic rate constants associated with the reactions on the anode and cathode side, and the changes in catalytic activity or active site density due to catalyst degradation. A third term deals with the decrease in the rate of mass transfer within the MEA. The resulting model is largely mechanistic, with most terms being derived from theory or including coefficients that have a theoretical basis, but includes empirical parameters to deal with the changing performance. Changes in the polarisation curve predicted by the generalised steady state electrochemical degradation model (GSSEDM) are demonstrated from the data for the performance of typical PEM fuel cell hardware.