Additives have been proven to be useful in improving electrospinnability and controlling fiber morphology through the modification of solution properties, including the conductivity, viscosity, and surface tension. In this study, the effects of adding small amounts of four different types of ionic liquids [i.e., 1-butyl-3-methylimidazolium chloride (C4MIMCl), 1-dodecyl-3-methylimidazolium chloride (C12MIMCl), 1-ethyl-3-methylimidazolium bromide (C2MIMBr), and 1-ethyl-3-methylimidazolium phosphate (C2MIM)(3)PO4] on the solution properties, electrospinning process, and characteristics of polyacrylonitrile (PAN) were investigated. The results show that the solution conductivities significantly increased with the addition of different ionic liquids with concentrations varying from 0.1 to 1.0 wt %, and the tendency depended on the structures of the ionic liquids. (C2MIM)(3)PO4 showed the highest conductivity value; this was followed by C2MIMBr, C4MIMCl, and C12MIMCl. The ionic liquids formed visible crystals; this made the fiber surfaces rough, and some fiber segments underwent partial aggregation. A regular varying tendency between the minimum mean diameter of the PAN/ionic liquid fibers and the structure of the ionic liquid was found. The PAN/N,N-dimethylformamide (DMF)/(C2MIM)(3)PO4 solution showed the highest conductivity among the four systems with different ionic liquids added, and the thinnest minimum diameter of the PAN/(C2MIM)(3)PO4 fibers appeared with a relatively low ionic liquid concentration of 0.25 wt %, whereas the PAN/DMF/C12MIMCl solution had the lowest conductivity, and the minimum mean diameter of PAN/C12MIMCl fibers appeared at a relatively high ionic liquid concentration of 0.8 wt %. Although the conductivity of the PAN/DMF/C2MIMBr solution was higher than that of the PAN/DMF/C4MIMCl solution, the minimum mean diameters of the PAN/C2MIMBr and PAN/C4MIMCl fibers appeared at the same ionic liquid concentration of 0.5 wt % because of the similar ionic activities of C2MIMBr and C4MIMCl. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2359-2368, 2013