Abstract: A continuous process for the separation of a mixed xylene isomer feedstock into o - and p - isomer fractions by feeding the mixed xylene isomer feedstock to a fractional distillation column in which a fraction containing a major portion of the o-xylene in the feedstock is removed from the bottom of the column and further fractionated to recover substantially pure o-xylene and a major portion of the p-xylene in the feedstock is removed as an overhead fraction, the p-xylene being removed from the overhead fraction by crystallisation and the mother liquor therefrom being recycled to the feed after contact with an isomerisation catalyst the improvement which comprises contacting the mother liquor remaining after crystallization of p-xylene, together with additional mother liquor, with a substrate active for the selective adsorption of p-xylene and thereafter recovering the adsorbed p-xylene from said substrate, the amount of mother liquor added being substantially equal to the quantity of p-xylene recovered from