Thermal unfolding of two forms of Aspergillus niger glucoamylase was observed by adiabatic differential scanning calorimetry (DSC) at pH 7. The DSC traces of the larger form of the enzyme, G1, and the shorter form which lacks the C-terminal starch-binding domain of G1, G2, could be resolved by assuming two-state unfolding of five and four independent components, respectively. The thermal unfolding of the starch-binding domain was found to be reversible, but that of the catalytic domain was irreversible. The DSC observation of G1 and G2 in the presence of β-cyclodextrin or 1-deoxynojirimycin showed that there was no domain interaction between the starch-binding domain and catalytic domain.