Selective adsorption of xylene isomers was investigated by measuring the breakthrough curves of the X- and Y-zeolites ion-exchanged with K⁺ and Ba⁺⁺. KY-zeolite showed the best sorption selectivity for p-xylene due to its reduced acidity. Also the effects of adsorption temperature and moisture contents on the selectivity were discussed. The adsorption equilibrium constant of p-xylene was the largest and the heats of adsorption for each xylene isomer were estimated to be roughly the same.