Modulated temperature techniques allow to separate the reversing and non-reversing contributions of material transitions. To investigate reversible crystallization and melting of isotactic polypropylene (iPP) at microstructural level, in this research, modulated temperature Fourier transform infrared (MTFTIR) and quasi-isothermal FTIR (QIFTIR) analyses are used. By following the intensity variation of iPP regularity bands, associated with 3(1) helix structures of different lengths (n repeating units), MTFTIR evidences that, independently from helix length, a reversing coil-helix transition takes place few degrees below the non-reversing crystallization onset. By comparing spectroscopic and differential scanning calorimetry experiments performed in quasi-isothermal conditions, the reversing transition was found to be associated with the reversible melting-crystallization phenomenon. Moreover, QIFTIR evidences that helices of different lengths contribute differently to the reversible transition: the helices composed of n = 10 and n = 12 are active into all the explored temperature range (30-130 degrees C) whereas the shortest (n = 6) and the longest (n > 15) helices contribute to reversibility at T > 100 degrees C. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 922-931