The selective adsorption capacities of NaY-and KY-zeolites for xylene isomers and toluene were investigated by measuring the rate of unidirectional diffusion in a batch adsorber and the breakthrough behavior in a fixed-bed column. The suquence of sorption selectivity was found as m-xylene > toluene > p-xylene with NaY-zeolites and as the converse with KY-zeolites. The reversal of selectivity could not be explained by the molecular sieving effects but by the interactions between adsorbing molecules and cation as well as among adsorbing molecules themselves.