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Thermal inactivation of glucose oxidase (GOD; ␤-Dglucose: oxygen oxidoreductase), from Aspergillus niger, followed first order kinetics both in the absence and presence of additives. Additives such as lysozyme, NaCl, and K 2 SO 4 increased the half-life of the enzyme by 3.5-, 33.4-, and 23.7-fold respectively, from its initial value at 60°C. The activation energy increased from 60.3 kcal mol ؊1 to 72.9, 76.1, and 88.3 kcal mol ؊1 , whereas the entropy of activation increased from 104 to 141, 147, and 184 cal⅐mol ؊1 ⅐deg ؊1 in the presence of 7.1 ؋ 10 ؊5 M lysozyme, 1 M NaCl, and 0.2 M K 2 SO 4 , respectively. The thermal unfolding of GOD in the temperature range of 25-90°C was studied using circular dichroism measurements at 222, 274, and 375 nm. Size exclusion chromatography was employed to follow the state of association of enzyme and dissociation of FAD from GOD. The midpoint for thermal inactivation of residual activity and the dissociation of FAD was 59°C, whereas the corresponding midpoint for loss of secondary and tertiary structure was 62°C. Dissociation of FAD from the holoenzyme was responsible for the thermal inactivation of GOD. The irreversible nature of inactivation was caused by a change in the state of association of apoenzyme. The dissociation of FAD resulted in the loss of secondary and tertiary structure, leading to the unfolding and nonspecific aggregation of the enzyme molecule because of hydrophobic interactions of side chains. This confirmed the critical role of FAD in structure and activity. Cysteine oxidation did not contribute to the nonspecific aggregation. The stabilization of enzyme by NaCl and lysozyme was primarily the result of charge neutralization. K 2 SO 4 enhanced the thermal stability by primarily strengthening the hydrophobic interactions and made the holoenzyme a more compact dimeric structure.