Connection between kinetics of hydrogen evolution reaction on iron and concentrations of hydrogen ions (at constant ionic strength equal to 1) and guanidine has been studied in the aqueous (hydrogen ions discharge is rate determining stage) and ethylene glycol + water (10 wt.% H2O, recombination of adsorbed hydrogen is rate determining stage) acidic chloride solutions. Guanidine (NH2)(2)CNH is entirely protonated in the studied conditions. But in the aqueous solutions, a rate of its cathode reduction is insignificant and main proton donor is H3O+-ions. In ethylene glycol solutions a rate of guanidine ions discharge is larger, that though does not determine a nature of rate determining stage. Observed regularities are compared with hydrogen diffusion rate i(H) through the carbon steel membrane. i(H) value insignificantly depends on guanidine concentration in aqueous media, especially in the range of its small concentrations. In the ethylene glycol media such a dependence is considerable up to 1 mM guanidine concentration.. Nature of rate determining stage of hydrogen evolution reaction does not determine a character of i(H) = f(C-HCl) and i(H) = f(C-guanidine) dependences. (C) 2011 Elsevier By. All rights reserved.