An aqueous two-phase system (ATPS) comprised of polyethylene glycol (PEG) and potassium salts was used to partition an extracellular xylanase produced by the thermophilic fungus Melanocarpus albomyces IIS-68. The enzyme accumulated in the top (PEG) phase. Yield (YT) in the top phase was observed at different volume ratios (V-T/V-B) and a partition coefficient calculated.