Two methods used in the literature to calculate the surface concentration of adsorbed species on an electrode from experimental results obtained by the ＇electrode lowering＇ technique are compared. It is shown that the values of surface concentration calculated by the use Of the theoretical equations proposed by Wieckowski et al. are appreciably lower than those calculated by the use of the calibration plot. It is suggested that this discrepancy is caused mainly by the difference in the counting efficiency for the backscattered and impedient radiation which had not been taken into account in the derivation of the theoretical formula for the surface concentration.