A detailed investigation of the reorganization processes occurring in the crystalline phase of poly(butylene terephthalate) (PBT) during fusion was performed by temperature-modulated differential scanning calorimetry (TMDSC). Quasi-isothermal TMDSC analyses were conducted for extended times at different temperatures in the whole melting range, revealing different degrees of reversible melting. The latent heat involved in the fusion/recrystallization/crystal-perfection processes was found to decrease with time for the quasi-isothermal analyses performed at temperatures below 500 K. At higher temperatures, on the contrary, recrystallization prevailed with a continuous increase of the apparent reversing heat capacity as a function of time. The multiple melting behavior of PBT, often reported in the literature, was correlated with the results of the present quasi-isothermal TMDSC investigation.