A series of regiorandom poly(3-alkylthiophenes) [P3ATs] with 4 <= C <= 12 alkyl carbons per side chain are studied by shear, calorimetry, X-ray scattering techniques, and infrared spectroscopy. We show that the tendency of alkyl groups to segregate in small alkyl nanodomains is a common feature of semicrystalline and amorphous members of this series. Semicrystalline poly(3-dodecylthiophene) (C = 12) samples show a pronounced lamellar structure with a coherence length of about 150 angstrom, corresponding to staples of main chain and alkyl nanodomains comparable to the findings for regioregular P3ATs, while the alkyl nanodomains in amorphous P3ATs have more irregular shape and boundaries. An additional relaxation process alpha(PE), which is related to the dynamics of CH2 units in the alkyl nanodomains, appears in amorphous samples with C >= 6. Its frequency temperature dependence is very similar to that of the alpha(PE) process found in other side-chain polymers containing alkyl groups with identical length. This is a strong argument Supporting the idea that main chains and alkyl groups separate in amorphous systems. Another interesting finding is the appearance of three distinct melting peaks for regiorandom poly(3-dodecylthiophene) [P3DDT] at temperatures between -10 and 50 degrees C. Isothermal crystallization experiments are performed, and possible reasons for the appearance of three distinct melting peaks are considered. Similarities and differences between regiorandom and regioregular P3ATs in the semicrystalline state are described and discussed on the basis of their microstructure.