Passivating films formed on steel working electrodes were characterised by the values and ratio of the rate constants (k(ox)(0) and k(red)(0)) of the charge transfer of the Fe(CN)(6)(3-)/Fe(CN)(6)(4-) redox couple. k(0) values were determined by a special CV technique in the solutions containing both the oxidised and reduced forms of the redox couple. These values were influenced by the chemical composition of steel and the chemical conditions of the solutions where the passive layers formed. The small reaction enthalpies of the formation of the passivating film on aluminum and steel were determined with a reaction microcalorimeter. These reactions are exothermic for both metals. In the case of aluminum the number of oxide monolayers was calculated on the base of a model including the factors that control the passivation process.