This paper presents results from an investigation of the permeation rate of gaseous hydrogen through a double-layer membrane by the classic gas accumulation method at 400 °C. Dense 8-micrometre thick tungsten films were grown on highly permeable 40 mm diameter Eurofer substrates by the combined magnetron sputtering and high energy ion bombardment method. By in-situ exposure of the tungsten side to pure oxygen at 0.9 bar for 2 h, a noticeable decrease in permeation rates at 400 °C was achieved, expressed as the permeation rate reduction factor, which ranged from 3.8 to 38. Oxide characterisation followed after permeation rate tests. Focused ion beam cross-section after the oxidation/permeation rate cycle revealed an oxide layer thickness between 80 and 100 nm. X-ray diffraction and Raman analyses revealed a mixture of many non-stoichiometric oxide types after the oxidation/permeation rate cycles. Their corresponding hydrogen permeability at 400 °C in the range from 8.1 × 10⁻¹⁸ – 5.7 × 10⁻¹⁷ mole H₂/s/m/Pa^0.5 was calculated from measured data.