Poly(3-hexylthiophene) (P3HT) as a conjugated polymer has many electrical and photovoltaic applications. However, reliable values for the equilibrium melting temperature (T degrees m) and for the enthalpy of fusion of a perfect P3HT crystal (Delta H degrees m) are still in doubt. In the published works, Delta H degrees m ranges from 33 to 99 J/g. Here, the melting point of P3HT in P3HT-diluent mixtures is measured with changing the diluent concentration. By satisfying the requirements of Flory relation, "working with the same polymer crystals" and "working in the equilibrium state," usually neglected in the literature, a reliable value of Delta H degrees m = 83 +/- 2 J/g is obtained. The reported values for T degrees m of P3HT are also in a wide range, 260 - 300 degrees C. In this work, accurate applying of the Hoffman-Weeks relation, against the effects of lamellar thickening, yields T degrees m = 272 +/- 2 degrees C rather than 300 +/- 2 degrees C reported in the literature. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 431-437