Abstract The importance of protective oxide scales as barriers against hydrogen and tritium permeation in nuclear process heat systems is demonstrated. Since it is not possible to predict the overall permeation rate under real conditions, an experimental facility was constructed to carry out these measurements. The results of the first 2000 h run demonstrate a considerable decrease in hydrogen permeation owing to the growth of protective oxide scales. A kinetic analysis suggests a parabolic time dependence. The long term residual permeation rate must be attributed to imperfections in the oxide layers. Interference effects from a certain oxidation of the primary gas surfaces are also considered. Finally, some remarks are made on relationships between hydrogen permeation and metallurgical diagnostics.