Semicrystalline poly(oxy-2,6-dimethyl-1,4-phenylene), PPO, is the first example of a polymer that has the glass transition of the rigid amorphous fraction (RAF) above the melting temperature of the crystals. Such phase structure does not allow fast melting below the glass transition and hinders recrystallization. The PPO is analyzed by temperature-modulated differential scanning calorimetry. In the standard DSC of PPO with about 30% crystallinity, a melting peak appears at 515 K, but no glass transition can be seen. The reversing heat capacity of the semicrystalline PPO shows no melting, but a glass transition at about 502 K. Annealing below the beginning of melting causes not only the development of a glass transition as known from amorphous PPO but also a reduction in crystallinity. The RAF was calculated from the deficiency between the measured, reversing heat capacity and the expected thermodynamic heat capacity calculated from our data bank.