This study focuses on the investigation of the electrochemical reaction along a novel cathode flow channel of PEM fuel cells with various shoulder/channel (S/C) ratios at the outlet port. A three-dimensional mathematical model, considering conservation principles of mass, momentum, species and electric current is employed. Local variations of important model variables such as reactant concentration and local current density are presented by contour plots to elucidate the effects of channel geometry on transport process, catalyst reaction and cell performance. The potential fields of solid and membrane phases are also resolved in the cell domain and the driving force of the electrochemical reactions - the catalyst activation overpotential - is harnessed in modeling. Numerical calculations reveal the influence of the cathode channel configuration on the local distributions of various model variables. The results also show the dependence between optimal channel configuration and cell operating condition. At a medium reaction rate, the reaction sites underneath the shoulder region generate more current than the channel region. Therefore, a convergent channel configuration with a larger S/C ratio at the outlet port develops more current because such a design facilitates the electron transport and enhances local activation overpotential. However, as the cell voltage decreases and the reaction rate increases, such configuration loses its merit gradually as the requirement for a higher reactant concentration is more important and the reaction sites underneath the channel region have a higher reaction rate. Consequently, the divergent channel configuration with a lower S/C ratio of 0.67 perfonns better at a cell voltage of 0.22 V (c) 2007 Elsevier B.V. All rights reserved.