When measuring the current due to the Oxygen Reduction Reaction (ORR) and hydrogen oxidation reaction (HOR) on Pt and Pt alloys in aqueous electrolyte, it is important to take care of two major sources of error that are relatively easy to correct for. First, when measuring ORR voltammetry, adsorption processes are superimposed on the current. Second, the system resistance causes an Ohmic drop that may have a profound effect on the measured curves, especially at the higher currents close to the diffusion limiting current. More importantly, we show that it also influences the kinetic part of the potential curve in such a way that the Tafel slope may be determined incorrectly when failing to correct for Ohmic drop. Finally, because electrolyte resistance lowers with increasing temperature, failure to compensate for Ohmic drop may lead to erroneous conclusions about the temperature-dependent activity of a catalyst as well as the corresponding activation energies. (C) 2010 Elsevier B.V. All rights reserved.