Abstract In a typical proton exchange membrane fuel cell (PEMFC), a gas crossover brings parasitic reaction, such as hydrogen and carbon oxidation at the cathode and oxygen reduction at the anode, which reduces open circuit potential (OCP) because of undesired potential mixing. Therefore, a two-dimensional computational fluid dynamics model was formulated to elucidate the variation of cell polarization, as the parameters affecting the mixed-potential effect change. The present model was validated by comparing the simulated cell polarization with experimentally measured cell polarization. The membrane electrode assembly was prepared by the decal transfer method, which gives uniform electrode formation. Model comparisons were also conducted to clearly state the significance of the fuel crossover and carbon oxidation reaction on OCP decrease. The results have shown that model prediction fits experimental data with an acceptable range of error, under two different relative humidity conditions of 50 and 100%. In addition, further investigations were conducted on (i) effect of gas permeation coefficient in membrane, (ii) effect of membrane thickness and (iii) effect of carbon oxidation and their influences on OCP and cell polarization are discussed.