In the present study, an aqueous two-phase system (ATPS) composed of polyethylene glycol (PEG) and ammonium sulphate ((NH4)(2)SO4) was used to purify a thermostable xylanase produced by the thermophilic fungus Paecilomyces thermophila J18 in solid-state fermentation (SSF). The effects of PEG, (NH4)2SO4, loading mass and pH on xylanase partition in ATPS were investigated. Xylanase partitioned in the top polymer-rich phase, while the remainder of proteins partitioned in the bottom salt-rich phase. The ATPS composition (w/w) of 12.5% PEG-4000, 25% (NH4)2SO4 and 50% enzyme solution at pH 7.2 was proved to be an excellent system for purification of xylanase in a single-step operation. SDS-PAGE analysis revealed that the purified xylanase was near homogeneity. A purification factor of 5.54 and a 98.7% enzyme yield were achieved in the top phase of this system. However, the xylanase produced in SSF was purified to homogeneity with a recovery yield of 34.5% by three steps. Therefore, ATPS provides an efficient and attractive method for purifying the xylanase in SSF. (c) 2007 Elsevier Ltd. All rights reserved.