The effect of trehalose (0.5 M) on the thermal stability of two endo-1,3(4)-beta-xylanases, alkaline Xyl I (pI > 9.3) and a neutral Xyl 11 (pI 7.5), purified from the culture broth of Bacillus subtilis CCMI 966 was studied at 60 and 70 degreesC. Thermal deactivation kinetics of Xyl I and Xyl 11 were analyzed in the absence and in the presence of trehalose. Suitable mechanisms were proposed to describe the deactivation process. Trehalose not only stabilized both enzymes at 60 and 70 degreesC, but also increased their initial activity. Addition of trehalose allowed both isoenzymes to attain a final state with residual activity. In contrast, without trehalose, the final state had no activity. At 60 and 70 degreesC, the Xyl I half-lives increased 2.5-and 2-fold, respectively, in the presence of trehalose. The half-life for Xyl 11 at 60 degreesC, increased about eightfold. At 70degreesC, the half-life increased about twofold and gave rise to a final state with residual activity of about 35%. (C) 2003 Elsevier Inc. All rights reserved.