A simple and efficient method to induce porosity both in the core and on the surface of electrospun submicrometer polymer fibers has been demonstrated by combining nonsolvent-induced phase separation with electrospinning. In this modified electrospinning process, fibers are collected in a bath filled with a nonsolvent for the polymer being electrospun. The presence of residual solvent in the nanofibers causes phase separation once the fibers reach the nonsolvent bath. Poly(acrylonitrile) (PAN) in dimethylformamide (DMF) is chosen as the model polymer/solvent system. The versatility of the approach is demonstrated by extending the technique to poly(styrene)/DMF, poly(styrene)/toluene, and poly(methyl methacrylate)/DMF systems. With a suitable solvent (ethanol) and optimized tip-to-collector distance, the specific surface area of the porous PAN fibers increased to an order of magnitude compared to that of the smooth fibers obtained by the conventional electrospinning. Further, this electrospinning technique is extended to core-shell electrospinning, enabling the fabrication directly in one step of PAN-based hollow fibers having porosity both in the surface and the bulk.