Abstract: A new composition of matter, a method for the preparation of a rubidium exchanged Type X structured zeolite and a separation process employing the same. The zeolite adsorbent is prepared by a procedure employing the aqueous ion exchange step and a final activation step including contacting a sodium-X zeolite with a rubidium salt in aqueous solution. The process involves the selective separation of ethylbenzene from a C.sub.8 aromatic isomer mixture containing ethylbenzene and employs the use of the rubidium exchanged type X structured zeolite in an adsorption separation process.