Stabilization mechanism of chloride ion on thermal denaturation of Arthrobacter sarcosine oxidase (SoxA) was investigated by calorimetric assays. The thermal unfolding of SoxA observed by differential scanning calorimetry was apparently irreversible. The temperature of unfolding was higher as higher concentration of chloride ions. The temperature of unfolding in the presence of chloride ions at a concentration of 1 M increased by 14℃, compared to the case in the absence of chloride ions. Isothermal titration calorimetry showed that a SoxA molecule bound a chloride ion with the dissociation constant of 86 mM. These calorimetric results suggested that a simplest mechanism that can explain the thermal unfolding of SoxA in the presence of chloride ion is as follows: NCl? →← U + Cl? and U → UF, where N, U, and UF represent SoxA in a native, reversibly unfolded, and irreversibly unfolded state, respectively. In this model, SoxA undergoes reversible unfolding with simultaneous dissociation of a chloride ion and subsequently the irreversible process occurs. The addition of a chloride ion shifts the equilibrium to favor the native-state side, resulting in an apparent stabilization.