Xylanase was produced by growing Thermomyces lanuginosus DSM 10635 in different carbohydrate containing media. The xylanase was purified to electrophoretic homogeneity and some of its enzymatic properties were studied. The molecular mass of this xylanase estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was 25.5 kDa, and the pI was approximately 3.7. Applying mass spectrometric analysis, the average molecular mass of the xylanase was determined to be 21295.17 Da, which is practically the same as the theoretical molecular mass of the T lanuginosus DSM 5826 xylanase (crystal structure lyna). Accordingly, the thermostability and enzymatic properties of DSM 10635 xylanase were similar to DSM 5826 xylanase. The inactivation of DSM 10635 xylanase during the elevation of temperature proceeded slowly at pH 6.5 and 8.0, whereas at pH 5.0, the inactivation profile was steep, indicating that changes in the electrostatic interactions decrease the stability at lower pH. The thermostabilizing effect of the substrate on DSM 10635 xylanase was studied and it was observed to be significant only under acidic conditions. The half-life in the presence of 1% birchwood xylan (w/v) was 7-fold, 4-fold and 1.5-fold higher than without the substrate at pH 4.0, 5.0 and 6.5, respectively. No increase in the stability by the presence of substrate was observed at pH 9.0. (C) 2004 Elsevier Inc. All rights reserved.