Detailed study of heat and mass transfer inside a silica-gel packed bed, especially extraparticle diffusion for adsorption of water vapor, was carried out in a densely packed adsorbent bed, which normally prevails for realizing AHP technology. Experimental data for temperature and pressure distributions in the packed bed of adsorbent for adsorption of water vapor on silica gel is evaluated in terms of heat and mass transfer numerical calculation based on the model of water vapor transport which accounts for both heat and mass transfer of the interparticle void and the adsorbent particle. Subsequently, the effect of particle size and the rate of adsorption are discussed from the viewpoint of adsorption mechanism. It is found that the model calculation was appropriate since the results were found to agree with the experimental values. For the experimental conditions, mean adsorption rate is found to increase with particle size under d(p) less than or equal to 500 mu m, above which the adsorption drastically decreases with an increase in particle size, From this result, it is concluded that there is an optimum value of particle size relative to the bed height for attaining a maximum adsorptivity.