High‐precision differential scanning calorimetry (DSC) and circular dichroism (CD) have been employed to study the thermal unfolding of chitinase 40 (Chi40) from Streptomyces thermoviolaceus. Chi40 belongs to family 18 of glycosyl hydrolase superfamily bearing a catalytic domain with a “TIM barrel”‐like fold, which exhibits deviations from the (β/α)₈ fold. The thermal unfolding is reversible at pH = 8.0 and 9.0. The denatured state is characterized by extensive structural changes with respect to the native. The process is characterized by slow relaxation kinetics. Even slower refolding rates are recorded upon cooling. It is shown that the denaturation calorimetric data obtained at slow heating rate (0.17 K/min) are in excellent agreement with equilibrium data obtained by extrapolation of the experimental results to zero scanning rate. Analysis of the DSC results reveals that the experimental data can be successfully fitted using either a nontwo‐state sequential model involving one equilibrium intermediate, or an independent transitions model involving the unfolding of two Chi40 energetic domains to intermediate states. The stability of the native state with respect to the final denatured state is estimated, ΔG = 24.0 kcal/mol at 25°C. The thermal results are in agreement with previous findings from chemical denaturation studies of a wide variety of (β/α)₈ barrel proteins, that their unfolding is a nontwo‐state process, always involving at least one unfolding intermediate. Proteins 2006. © 2006 Wiley‐Liss, Inc.