To permit more accurate calculations of tritium permeation for design purposes, theoretical expressions for quasiunidirectional ordinary diffusion of hydrogen isotopes through nonisothermal plane, cylindrical shell, and spherical shell barriers are presented. Arrhenium-type dependence of mass diffusivity on temperature and steady-state constant-thermal-conductivity temperature gradients through the barriers are considered. Analyses that consider thermal diffusion with both constant and temperature-dependent heat of transport are also presented. Other topics discussed are amounts of dissolved hydrogen, variable-thermal-conductivity temperature profiles, hydrogen isotope trapping by chemical impurities, crystal lattice imperfections, and grain boundaries, and mixing rules for dilute dissolution of hydrogen isotope mixtures in metals. Numerical results are given which reveal that neglect of thermal diffusion can lead to errors of up to several hundred percent in calculations of hydrogen isotope transfer through reactor components having large temperature gradients through them. Calculations of combined ordinary and thermal diffusion that rigorously treat the temperature dependences of thermophysical properties typically yield results that differ by only a few percent from results based on physical property evaluations at the arithmetic average of barrier face temperatures.