We present a review of our recent studies oil the accessibility of simple gases (Ar, N-2, CH4 and CO2) in disordered microporous carbons using transition state theory (TST) and molecular simulation techniques. A realistic carbon model rather than the slit-pore approximation is utilised, providing more accurate understanding, of complex adsorption equilibrium and dynamics behaviour at the molecular level in porous carbons, especially kinetic restriction of adsorbate molecules through highly constricted pore mouths of coals and molecular sieve carbons (MSC). This kinetic restriction leads to a molecular sieving effect which plays a vital role in gas separation using the MSCs. In particular, the realistic carbon model of a saccharose char used in a recent study was obtained by hybrid reverse Monte Carlo simulation. The time of adsorption or desorption of the single gas molecule between two neighbouring pores through a highly constricted window of the realistic saccharose char model was determined using TST. Finally, the validation of TST calculated results of adsorption and desorption times against experimental measurements as well as molecular dynamics Simulation is also presented in this article. (C) 2009 Curtin University of Technology and John Wiley & Sons, Ltd.