The enthalpy of fusion for a perfect, infinite poly(3-hexylthiophene) (P3HT) crystal (Delta Hm infinity) must be known to evaluate the absolute crystallinity of P3HT. This value, however, is still ambiguous as different values have been reported using various experimental techniques. Here, we extrapolate the enthalpy of fusion for extended chain crystals of oligomeric P3HT to infinite molecular weight and obtain a value of Delta Hm infinity approximate to 42.9 +/- 2 J/g employing differential scanning calorimetry with a correction based on grazing incidence small angle X-ray scattering data. Also, we define the onset of chain folding within P3HT crystallites at a chain length of 5 Kuhn segments. Knowledge of Delta Hm infinity allows calculation of P3HT percent crystallinity in thin films for applications such as organic field effect transistors and solar cells. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1469-1475