The overall crystallization rates of poly (3-hexyl thiophene) (P3HT) cocrystals with different amount of regioregularity of the components are measured using differential scanning calorimetry (DSC). Two pairs of cocrystals with varying compositions of the component polymers (viz P3HT(R) (regioregularity 92 mol %)/P3HT-2 (regioregularity 82 mol %), and P3HT-2/P3HT-1 (regioregularity 75 mol %)) are used. The crystallization rate at the same isothermal crystallization temperature (T-c) decreases with increasing regioregularity. The low Avrami exponent values (0.15-1.0) in all the samples suggest the presence of rigid amorphous portion, which cannot diffuse out quickly from the crystal growth front (soft impingement). Analysis of crystallization rate using Laurintzen-Hoffman (L-H) growth rate theory indicates Regime I to Regime 11 transition in all the samples. The product of lateral and end surface energy values (sigma sigma(c)) increases gradually with increasing regioirregularity in the blend. Analysis of or values indicates chain extension of the components in the melt of the blend and the entropy of activation (Delta SI-II) of the cocrystals are higher than those of the components. The entropy of cocrystallization (Delta S-c) values are 1-2.4 e.u for P3HT(R)/P3HT-2 system and 0.5-1 e.u for P3HT-2/P3HT-1 system. (c) 2006 Wiley Periodicals, Inc.