The cloud-point curves for solutions of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) in o-dichlorobenzene as a function of composition were measured using a thermooptical analysis method. The binary polymer solution shows phase equilibria between a single coiled polymer in solution and polymer in nanocrystalline aggregation, in which the coexistence points at the melting temperature (i.e., solid-liquid equilibria) could be described by the theory of melting point depression combined with the Flory-Huggins model. Through the correlation of theory and experimental data, the heat of fusion per crystalline repeat unit (Delta H-u = 47.5 J/g) and the binary interaction parameter (chi = 32.6/T) were estimated. When the solutions were further characterized using UV-vis spectroscopy and X-ray diffraction (XRD), the absorption spectra showed a red-shift (lambda(max) = 464 to 518 nm) with increasing P3HT concentration from 0.01 to 6 wt %, and the XRD data displayed a trace of (100) peaks at 2 theta = 5.39 degrees with d-spacing of 1.6 nm from lamellae associated with interdigitated hexyl side chain packing. Here, the P3HT's crystallite size of similar to 6 nm was also calculated from the Scherrer equation. Finally, the charge transport behavior in P3HT-based field-effect transistors was investigated as a function of solution concentration. Resultantly, the hole mobility of 1.03 x 10(-2) cm(2)/(V s) was measured on a film spin-coated from 2 wt % P3HT in DCB phase-separated solution.