Isotactic polystyrene (iPS) is demonstrated to have unique thermal behavior, showing dual reversible crystal melting and irreversible enthalpic relaxation of its rigid amorphous fraction (RAF). (quasi-isothermal temperature-modulated differential scanning calorimetry (TMDSC) and standard DSC were used to study the heat capacity of cold-crystallized iPS. IPS shows two or three endotherms depending upon cold crystallization temperature, T-c. Crystal melting causes the higher temperature endotherm(s), and under quasi-isothermal conditions, we report for the first time observation of dual locally reversible melting endotherms. Quasi-isothermal TMDSC shows that the RAF is established at the crystallization temperature in iPS. Furthermore, we show that the lowest temperature endothermic peak, called the annealing peak, represents the transition of the RAF. For cold-crystallized iPS the annealing peak is an irreversible, enthalpy-involved relaxation of RAF, which transforms solidlike RAF into liquidlike mobile amorphous fraction. Depending upon T-c, RAF may be the sole contributor to the lowest temperature endotherm. To accommodate the relaxation of RAF, the experimentally determined heat capacity should be written in terms of the underlying linear baseline heat capacity plus the enthalpic terms relating to crystal melting and to relaxation of RAF.