Poly(3-dodecyl thiophene) (P3DDT) produces thermoreversible gels in o-, m-, and p-xylene (Xy). All of these gels have fibrillar network morphology and exhibit reversible first-order phase transitions. The enthalpy changes related to the polymer part of P3DDT-p-Xy gels indicate polymer-solvent complex formation having stoichiometry of P3DDT monomeric unit/p-Xy = 2:3. In P3DDT-o-Xy and P3DDT-m-Xy gels, the enthalpy changes also indicate similar polymer-solvent complexation of 2:3 type. In our experimental condition, only p-Xy crystallizes, and analysis of its enthalpy changes indicate the formation of a polymer-solvent complex of stoichiometry P3DDT monomeric unit/p-Xy = 3: 1. In the phase diagrams, there are main chain melting and side chain melting boundaries; the latter encompasses the whole composition region. In the P3DDTp-Xy system, there is an additional solvent melting boundary. The phase diagram nature suggests that the 2:3 complex is singular type. The solvent-subtracted Fourier-transform infrared (FTIR) spectra showed different absorbance peaks in the gels of different compositions and are attributed to the different polymer-solvent complexes produced in the gels. Wide-angle x-ray scattering (WAXS) study indicated an increase in the lamellar thickness with decreasing P3DDT concentration in the gel, and the intensity ratio of the diffraction peaks also changes from those of the polymer melt. The lamella structure is absent in the dilute gel (W-P3DDT = 0.14) indicating that dilute gels have different structure than concentrated gels (W-P3DDT = 0.59 and 0.86) have. Molecular modeling using MMX program supports the increase of interchain lamellar thickness of P3DDT chain with incorporation of p-Xy molecules.