This study investigates morphology control of polymer nanofibers produced from solutions of two immiscible polymers in mutually miscible solvents by gas jet fiber (GJF) method. The morphology is controlled by selecting the solvents with a range of vapor pressure and solubility parameter to govern respectively the evaporation rates and the polymer-solvent affinity. Bi-component nanofibers with core-shell, bi-lobal, and interpenetrating network (IPN) morphology are produced by evaporation-induced phase separation of the polymers during spinning of homogeneous solutions of hydrophobic polyvinyl acetate (PVAc) and hydrophilic polyvinyl pyrrolidone (PVP) in two mutually miscible solvents. The tendency of phase-separation is inferred from the values of polymer-solvent mixture affinity. The results indicate that solvent pairs with closely matched vapor pressure yield IPN morphology, while solvents with large differences in vapor pressure produce core-shell morphology. Bi-lobal morphology is produced for solvent pairs with moderate differences in vapor pressure. In each case, nanofibers of smaller diameter are obtained when solvent pairs of lower vapor pressure are used. (C) 2016 Elsevier Ltd. All rights reserved.