The equilibrium cocrystal formation of poly(3-alkyl thiophene) (MAT) blends has been studied by isothermal cocrystallization in a differential scanning calorimeter (DSC-7). The equilibrium melting points (T-m(0)) of the cocrystals are measured using the Hoffman-Weeks extrapolation procedure. The equilibrium phase diagrams are of three different types: (a) concave upward, (b) linear, and (c) linear with phase separation at higher content of lower melting component. The phase diagram nature depends on the regioregularity difference and also on the difference in the number of carbon atoms in the pendent alkyl group of the components. The origin of biphasic nature of type "c" phase diagram has been explored from the glass transition temperature (T-g) measurement using a dynamic mechanical analyzer. The biphasic compositions show two glass transition temperatures (T-g) as well as two beta transition temperatures (T-beta). The T(g)s of phase-separated regions correspond to almost the component values but the T(beta)s correspond to that of a lower (T-beta) component value, and the other is higher than that of the higher (T-beta) component value. Possible reasons are discussed from the interchain lamella thickness in the MAT blends and molecular modeling using molecular mechanics program.