We report the first measurements of the kinetics of adsorption on the very outermost surface sites of a porous material compared to measurements made of adsorption on the interior sites. NH3 and CO were employed in this study as representative of slow diffusion and fast diffusion, respectively, through porous TiO2. Adsorption of NH3 at 200 K occurs mainly at the very near surface (similar to 20 nm) region as observed by photoluminescence (PL) spectroscopy, and its distribution by surface diffusion through the powder is highly retarded as judged by transmission IR spectroscopy. In contrast, the adsorption of CO in the near-surface region at 120 K is followed by the fast distribution of CO by surface diffusion into TiO2 powder, causing the near-surface CO coverage to lag behind the coverage in the bulk. In the desorption process, the near-surface region delivers adsorbed CO molecules into the gas phase, accompanied by the supply of diffusing CO molecules from the interior. As a result, the adsorption/desorption processes for CO in the near-surface region of porous TiO2 show a pronounced hysteresis effect. As surface diffusion is retarded at lower temperatures, the hysteresis effect gradually disappears.