Hydrogen trapping sites have a great influence oil the hydrogen permeation through a slab sample. The diffusion of the hydrogen in a crystal is generally governed by a parabolic partial differential equation: a numerical Simulation code, realized for the study of the permeation of hydrogen in presence of trapping sites, has been utilized for the analysis of the influence of reversible and irreversible traps on the diffusion of hydrogen in a metal for the case of high (not negligible) hydrogen concentration with boundary conditions which cannot be treated analytically. (c) 2005 Elsevier B.V All rights reserved.