In this study, NN-dimethylformamide (DMF), which is a non-volatile solvent, was selected to dissolve polystyrene (PS) (190,000 g/mol) and the concentration of the PS solution (similar to 35 wt%) was maximized to retard the evaporation of the solvent under electrospinnable conditions. Unlike the porous morphology observed on the surface of the fibers electrospun from the PS solution in tetrahydrofuran (volatile solvent), the electrospun PS fibers obtained from the highly viscous solution of PS in DMF exhibited an intriguing surface morphology with numerous protuberances. The contact angle measurements indicated that the electrospun fibrous membranes were superhydrophobic with a water contact angle of 154.2 +/- 0.7 degrees. This superhydrophobicity was attributed to the combined effects of the regular nanostructural protuberant morphology formed on the surfaces of the individual fibers during electrospinning and the microstructural surface roughness of the electrospun membrane itself. (C) 2007 Elsevier B.V. All rights reserved.