In a large variety of microporous materials such as high-surface area catalytic solids, natural or polymer fibers, zeolites, porous sorbents, and other media, the diffusional mass transport of molecules is accompanied by immobilization on internal surface sites. The standard formalisms for diffusion, such as the Fick equation, require modification in order to distinguish between effects of the stochastic diffusion phenomenon (the molecular diffusivity) and consequences related to the immobilization kinetics. This can lead to corrections of several orders of magnitude to the apparent diffusivities derived from non-steady state techniques, such as uptake rate measurements. The modifications to the classical formalisms are relatively simple when the diffusion process is slow compared to the kinetics of ad-/desorption (immobilization). This is the case in many situations and technologies. The results have important implications in zeolite technology, molecular shape-selective catalysis, and many technologies employing microporous substances. Classical considerations of physical chemistry also provide guidance to conditions and systems where interpretation of transport in terms of diffusion coefficients may not be valid.