A semibatch constant molar flow-rate technique developed earlier by Do was applied to a series of light paraffins adsorbing onto a number of adsorbents. The adsorption and diffusion of methane, ethane, propane and n-butane on activated carbon at different temperatures were studied to determine the diffusivity of surface diffusion and its activation energy. It was found that the surface diffusivity decreases with the molecular weight in an exponential manner and increases with temperature according to the Arrhenius law. The ratio of the activation energy for surface diffusion to the heat of adsorption increases with the carbon number A mechanism for surface diffusion is snggesten to explain this increase.