The concentration of the mixture changes during the drying process of porous materials containing binary mixture, when a component is removed preferentially, so called the selectivity of the drying process. The selectivity depends on the phase equilibria, gas-and liquid-side mass transfer for given drying conditions. For this reason the less volatile component can be removed preferentially during the drying of porous materials containing binary mixture with azeotrope point. The liquid-side mass transfer rate of the constant rate period differs from that of the falling rate period. The liquid-side mass transfer rate depends on the characteristic of the porous materials(size and length of the capillary). The distribution of the concentration and the content of the binary mixtures in the porous materials were determined to investigate the influence of the liquid-side mass transfer on the selectivity.