The thermal unfolding of potato serine protease inhibitor (PSPI), the most abundant protease inhibitor group in potato tuber, was measured using far UV CD spectroscopy, fluorescence spectroscopy, and DSC. The results indicate that the thermal as well as the guanidinium-induced unfolding of PSPI occurs via a non-two-state mechanism in which at least one stable intermediate is present. Additionally, the occurrence of aggregation, especially at low scan rates, increases the apparent cooperativity of the unfolding and makes the system kinetically rather than thermodynamically controlled. Aggregate formation seems to occur via a specific mechanism of which PSPI in a tetrameric form is the end product and which may involve disulfide interchanges.