High-pressure sorption kinetics for N₂ and CO₂ in as-received TCM carbon, introduced in Part 2 of this series (J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros, J. Chem. Soc., Faraday Trans. I, 1989, 85, 1545), is elaborated here. Fickian processes are apparent for both gases, and provide the background against which the following complicated transport phenomena are overlaid. The most dramatic of these additional phenomena is CO₂-induced constriction-dilation, which causes a decrease over four orders of magnitude in the equilibration time as pressure increases from 0 to 60 atm. Desorption equilibration times at lower pressures following the CO₂ exposure also reflect residual dilation. Higher desorption rates compared to adsorption rates were also observed for N₂; however, the differences are much less extreme and are believed to be due to immobilization of adsorbate near constrictions instead of constriction-dilation. Evidence is presented to indicate the existence of a weak, transient barrier at the pore entrances at the beginning of sorption runs if appropriate conditions are chosen for each of the two gases.