[en] The theory of hydrogen permeation through metals is extended from the domain of high upstream H₂ pressures p₁, where diffusion limits the permeation flux phi, down to that of low p₁ where hydrogen penetration and recombination at the surface are rate determining. In the high p₁ case, as known, measurements yield the bulk parameters Ksub(s) and D(solubility and diffusion constants). At the low and intermediate pressures discussed, values of ksub(s) and ksub(r) can be determined: these phenomenological rate constants characterize respectively the dissociative solution and recombinative H₂ release processes. A dimensionless number W' related to surface and bulk parameters allows one to evaluate the transition pressure from high to low p₁ domains (W' =1). The variations of phi with p₁ (stationary case) and with time (nonstationary evolution), as calculated and observed in the experimental device described here, are in good agreement. In the high p₁ domain the results are close to data from the literature. (Auth.)