The melting of poly(ethylene terephthalate) is analyzed by quasi-isothermal, temperature-modulated differential scanning calorimetry. The measurement is done by sinusoidally changing the temperature in the melting range (+/-1.0 K). In the melting range a small portion of the sample melts reversibly. This observation is taken as a direct observation of the reversibility of melting of specific macromolecules as long as they are melting only partially and need no molecular nucleation for recrystallization.