We have developed a device which permits the plotting of time lag curves from test pieces subjected to various tension and compression stresses. The characteristic diffusion coefficients calculated from these experimental curves vary appreciably as function of the value and of the direction of the stress applied. the regular variation of these coefficients in the elastic region is followed by a discontinuity in the transition into the plastic region. The study of these variations makes it possible : - to demonstrate the hydrogen-dislocation interaction; - to define the role played by imperfections in the diffusion process; - to predict, as a function of the stress applied, the susceptibility of materials to an increase in brittleness due to hydrogen.