Core-sheath nanofibers with conductive polyaniline as the core and an insulating polymer as the sheath were prepared by electrospinning of blends of polyaniline with either polystyrene or polycarbonate. These unique core-sheath structures offer potential in a number of applications including nanoelectronics. When polyaniline was blended with poly(methyl methacrylate) and poly(ethylene oxide), only isolated domains of polyaniline in beadlike structures were formed. The phase morphology of electrospun fibers is thought to be dependent on the high-surface tension of the solution and the molecular weight of the polymers. Incompatibility of the polymers and low molecular weight of compositions played a key role in the formation of core-sheath structures, as opposed to co-continuous morphologies. TEM image of electrospun polyaniline/polystyrene nanofiber after staining by OsO4. The dark regions are polyaniline.