Electrospinning is a fiber spinning technique used to produce nanoscale polymeric fibers with superior interconnectivity and specific surface area. The fiber diameter, surface morphology, and mechanical strength are important properties of electrospun fibers that can be tuned for diverse applications. In this study, the authors investigate how the humidity during electrospinning influences these specific properties of the fiber mat. Using two previously uninvestigated polymers, poly(acrylonitrile) (PAN) and polysulfone (PSU) dissolved in N, N-Dimethylformamide (DMF), experimental results show that increasing humidity during spinning causes an increase in fiber diameter and a decrease in mechanical strength. Moreover, surface features such as roughness or pores become evident when electrospinning in an atmosphere with high relative humidity (RH). However, PAN and PSU fibers are affected differently. PAN has a narrower distribution of fiber diameter regardless of the RH, whereas PSU has a wider and more bimodal distribution under high RH. In addition, PSU fibers spun at high humidity exhibit surface pores and higher specific surface area whereas PAN fibers exhibit an increased surface roughness but no visible pores. These fiber morphologies are caused by a complex interaction between the nonsolvent (water), the hygroscopic solvent (DMF), and the polymer. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1734-1744, 2011