The effects of thiourea and benzotriazole on the hydrogen permeation into iron were studied using a Devanathan–Stachurski electrochemical double cell equipped with a thin iron membrane. Hydrogen was cathodically generated on the entry side and fully oxidized on the exit side. The spectral analysis of both the electrochemical impedance on the entry side and the permeation transfer function between the ac current signals on the two faces of the membrane were recorded according to a method recently proposed. The modifications of the impedance diagrams and the transfer function diagrams induced by the presence of the organic additives were interpreted at the light of the mechanistic model of the transfer function which was recently derived. It is concluded that both additives inhibit the hydrogen evolution reaction, but thiourea promotes hydrogen penetration by inhibiting the recombination reaction, whereas benzotriazole adsorption inhibits the formation of the Hads and therefore their penetration into the metal matrix.