After reviewing relevant equations for the calculation of exchange current density, a new equation is derived for hydrogen electrode reactions to correct for the influences of the hydrogen concentration change in the vicinity to the electrode surface. This equation is able to describe the polarization curve shape in the small polarization region as well as to calculate the exchange current (density). The abilities of this equation are demonstrated by the data obtained with a Pt rotating disk electrode in 0.1 mol l(-1) KOH solution. The exchange current density at 298 K under 1 atmosphere hydrogen pressure is found to be 0.103 mA cm(-2) with an apparent activation energy of 33.5 kJ mol l(-1). At a constant temperature, the exchange current is found to be proportional to the square root of the hydrogen partial pressure in the solution. (C) 2009 Published by Elsevier B.V.