The adsorption of methane and carbon dioxide on activated carbon pellets have been studied by a semi-batch constant molar flow rate method, which was first proposed by Do (1995). In this method, a very low and constant flow of adsorbate was introduced into a pre-evacuated adsorption cell. The pressure of the cell was monitored as a function of time and then analysed to extract dynamic parameters such as D-app (the apparent diffusivity if the diffusion process is the transport mechanism) and k(d) (rate constant for desorption if the Langmuir kinetics controls the uptake). For the range of pressure of 0 to 5 Torr and the range of molar flow rate used of 2 x 10(-9) to 12 x 10(-9) gmol/s, the equilibrium isotherms for both methane and carbon dioxide are linear and no heat effect was observed, supporting the isothermal analysis of linear systems. The analysis of the experimental data of different particle size suggests that the diffusion process, rather than the Langmuir kinetics, is the controlling mass transfer mechanism, and the dual diffusion (pore and surface) mechanism adequately explains the adsorption rate of both methane and carbon dioxide. The surface diffusivities at 293, 303, and 323 K for methane are 1.3 x 10(-4), 1.5 x 10(-4) and 2 x 10-4 cm(2)/s, respectively, and those for carbon dioxide are 1.9 x 10(-5), 2.3 x 10(-5) and 3.2 x 10(-5) cm(-2)/s, respectively. The activation energies for surface diffusion are found to be half of the heat of adsorption at zero loading. The proposed technique has been proven to be a reasonably quick and reliable method in determining diffusivities of gases in porous materials such as activated carbon.