An analytical solution for mass diffusion in a spherical microporous particle experienced with a small step change of gaseous phase adsorbate concentration is obtained. The mass diffusion in the solid is assumed to be micropore diffusion controlled. Thermal effect and gas-side mass transfer resistance are considered. The governing equations are solved by using the Laplace transformation method. Three factors, alpha, beta and gamma are defined to govern the heat and mass transfer. alpha and beta are relevant to the thermal effect and gamma dominates the gas-side mass transfer resistance. The applicable ranges of three simplified models are discussed. A limiting solution with mass diffusivity approaching infinity is obtained. (C) 2000 Elsevier Science Ltd. All rights reserved.