Abstract We have successfully engineered a disulphide bridge into the N-terminal region of Trichoderma reesei endo-1,4-β-xylanase II (XYNII) by substituting Thr-2 and Thr-28 with cysteine. The T2C:T28C mutational changes increased the half-life in thermal inactivation of this mesophilic enzyme from ∼40 s to ∼20 min at 65 °C, and from less than 10 s to ∼6 min at 70 °C. Therefore, the N-terminal disulphide bridge enables the use of XYNII at substantially higher temperatures than permitted by its native mesophilic counterpart. Altogether, thermostability increased by about 15 °C. The kinetic properties of the mutant XYNII were maintained at the level of the wild type enzyme. Our findings demonstrated that a properly designed disulphide bridge, here within the N-terminal region of XYNII, can be very effective in resisting thermal inactivation.