The cocrystallization behavior of poly(3-alkylthiophene)s (P3ATs) of varying regioregularity and alkyl chain length was explored via differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) studies. For the regioregular samples both DSC and WAXS studies revealed that cocrystallization was limited to the difference of the alkyl chain length by two carbon atoms. On the other hand, MAT samples with the same alkyl chain length but different regioregularities cocrystallized with a difference of 17 mol % head to tail (H-T) regularity studied here. P3AT samples with varying regioregularity and alkyl chain length by two carbon atoms showed cocrystallization for a regioregularity difference of 7 mol %. The WAXS study indicated that cocrystals had crystallized within the same interchain lamella and noncocrystals had crystallized within the independent interchain lamella of the components. It also indicated the formation of type I P3AT crystals with a noninterdigited side chain during the cocrystallization by melt-quenched conditions for all the samples. The phase diagrams clearly revealed that in some systems at the lower melting component rich region there exist two phase regions. The conductivity of the cocrystals, in both the doped and undoped states, is either intermediate or lower than the line joining their component conductivity values.