It is shown, hydrogen evolution reaction (HER) on iron occurs according to Volmer-Tafel mechanism in acidic aqueous chloride solutions with constant ionic strength. Relation between o-fluorophenylbiguanidine (o-FPBG) concentration and the kinetics and the nature of rate-determining step (rds) of hydrogen evolution reaction (HER) on iron in acidic chloride media with constant ionic strength has been established. Kinetic parameters of HER - dE/dlogi(c), dE/dlogC(H+), (dlogi(c)/dlogCH(+))(E), d eta/dlogi(c), d eta/dlogC(H+) and (partial derivative logi(c)/partial derivative logC(H+))(eta) - have been obtained at different concentrations of o-FPBG and compared with theoretical values appropriate to Volmer-Tafel mechanism. An increase in the o-FPBG concentration provokes sequential change of the rds in the hydrogen evolution reaction according to scheme: slow discharge - slow recombination - slow surface diffusion - slow discharge. It is proved that in the background solutions the slow discharge is followed by chemical recombination, but hydrogen diffusion rate through a steel membrane practically does not depend on the rds nature. Observed phenomena are interpreted - from a position of existence of two adsorbed hydrogen forms - subsurface and raised above the surface - and difference in surface coverage by either of them. (C) 2012 Elsevier B.V. All rights reserved.