A computer program for prediction of mass and heat transfer in a rotary-type dehumidifier was developed. The theoretical model considered the mass transfer resistance to be accounted for by both surface and Knudsen diffusion effects. The theoretical model was used to investigate the influence of the adsorption properties on the air drying process. A comparison between adiabatic and isothermal breakthrough curves for silica gel and zeolite 5A showed that considerable improvement in the efficiency of the air drying process could be obtained by heat removal from the silica gel bed compared with insignificance of this effect in the case of zeolite 5A. The theoretical model was also applied to investigate the drying performance of various compositions of a combined bed composed of silica gel in the front part and zeolite 5A in the back part. The results demonstrated that a significant improvement in dehumidifier performance could be obtained by arranging a suitable proportion of silica gel to zeolite 5A in the combined bed in comparison with a bed containing only silica gel. However, the optimal composition of the combined bed has to be considered in the light of the fact that of zeolite has to be regenerated at 130-150°C compared with 70-90°C for silica gel.