The properties of regioregular(R) (98.5 mol % H-T) and regioirregular(I) (80.5 mol % H-T) poly(3-dodecyl thiophene)(P3DDT)-organically modified montmorillonite (om-MMT) clay nanocomposites obtained from solvent-cast and melt-cooled procedures are compared. The solvent-cast P3DDTI nanocomposites showed partially exfoliated clay structure but P3DDTR nanocomposites showed multistack exfoliation. Type I crystalline polymorph was produced in solvent-cast systems whereas melt-cooled P3DDTI samples showed mesomorphic structure. Storage modulus of P3DDTI nanocomposites increased with clay concentration showing a maximum increase of 255%. The UV-vis spectra showed blue shift of pi-pi* transition band and photoluminescence spectra indicated seven times increase of normalized intensity in solvent cast P3DDTI composites. DC conductivity and I-V characteristic curves showed increased insulating properties with om-clay concentration. The physical, mechanical, and optical properties of P3DDTI nanocomposites are more improved than that of P3DDTR nanocomposites and from their pristine polymers. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 155-167,2009