The purpose of this study is to examine the effects of exchange current density, as dependent on operating parameters, on the activation polarization in an HTPEM fuel cell. In line with this purpose, cell performance tests were performed for different relative humidity levels (from 0 to 20% in steps of 5%) and six different acid doping levels (4.26, 4.53, 4.95, 5.51, 7.24, 7.51 H3PO4/RPU PBI) at constant cell temperature (120 degrees C). The exchange current densities, as dependent on acid doping level and relative humidity, were determined by fitting a zero-dimensional steady-state mathematical model to the cell performance test results. Finally, to investigate the activation polarization in the cell, the exchange current densities obtained were incorporated within a numerical model. The results show that the effects of relative humidity on the exchange current density are relatively limited at all acid doping levels when the relative humidity is greater than 5%. The exchange current densities exhibit a decreasing trend with increasing acid doping level. Furthermore, it was found that the acid doping level and relative humidity have a strong influence on the anode overpotential, whereas no significant effect on the cathode overpotential was observed.