An accurate and particular description of isotope effects in hydrogen transport within structural martensitic steels is highly needed in nuclear fusion technology in order to describe the tritium-material interaction on the basis of the properties of the non-radioactive hydrogen isotopes (protium and deuterium). As a result, tritium transport investigation becomes technologically more feasible because a cost-effective radioactive device is not mandatory. Additionally, a precise isotopic description allows differentiating the behaviour of the fuel-components deuterium and tritium within the blanket structures in reactor operation conditions. A time-dependent gas-phase isovolumetric desorption technique has been used to evaluate the isotopic effects in the diffusive transport parameters of hydrogen in an 8% CrWVTa reduced activation martensitic steel in the temperatures range 423 to 892 K and driving pressures from 4·104 to 1·105 Pa. Experiments have been run with both protium and deuterium obtaining their respective transport parameters diffusivity (D), Sieverts' constant (Ks), permeability (Φ), the trap site density (ηt) and the trapping activation energy (Et).