A time-dependent gas-phase isovolumetric desorption technique has been used to evaluate the diffusive transport parameters of hydrogen isotopes in polycrystalline tungsten in the temperatures range 673 to 1073 K and driving pressures from 1.3 10(4) to 10(5) Pa. Experiments have been run with both protium and deuterium obtaining their respective transport parameters diffusivity (D), Sieverts' constant (K-s), the trap site density (N-t) and the trapping activation energy (E-t). Isotope effects on these transport parameters are analysed and modelled. Because the classical isotope relation for diffusivity has not been fulfilled, quantum-statistical vibration theory has been applied to model the isotopic relation. A congruent isotopic variation of diffusion parameters related to the type of microstructure, bcc, has been confirmed.