In conditions of the cathodic polarization the influence of the composition of the mixed OH-C(2)H(4)OH + H(2)O solvent on the hydrogen diffusion flux i(H) through a steel membrane has been studied at the electrolyte composition of x M HCl + (1 - x) M LiCl, containing 1 - 10 mM pyridine. Simultaneously in the same conditions the hydrogen evolution reaction mechanism and the rate determining step nature on iron have been determined. A fraction of H(ads) being removed from the metal surface by the volume solid phase diffusion was estimated with use of rho coefficient: rho = i(H)/i(C) where i(C) is the cathodic current density. A character of a dependence of rho and i(H) on the cathodic polarization is independent of the hydrogen evolution reaction rate determining step nature. The rho value decreases with increasing cathodic polarization and an acidity growth. The results have been interpreted taking into account the surface coverage with two (raised above the surface and subsurface) forms of adsorbed hydrogen. (C) 2009 Elsevier B.V. All rights reserved.