The electrospinning process uses electrical force to produce nanofibers. A charged droplet acquires a conical shape known as the Taylor cone and then becomes unstable. A charged jet emerges front the vertex and develops a spiral path due to the electrically driven bending instability, which makes it possible. in a small space, for the jet to elongate by a large amount and produce nanofibers. Evaporation and the associated solidification were identified as important factors that affect the diameter of electrospun nanofibers. In this study, the evaporation rate and solidification of the charged jet were controlled by varying the relative humidity during electrospinning of poly(ethylene oxide) from aqueous solution. As the relative humidity increased, the solidification process became slower, allowing elongation of the charged jet to continue longer and thereby to form thinner fibers. As the relative humidity increased from 5.1% to 48.7%, the diameter of the solidified fiber decreased from 253 run to 144 nm. As the relative humidity increased above 50%, beads formed on the thinner fibers, indicating that the capillary instability Occurred before the jet solidified. The vapor concentration of solvent is an effective electrospinning process control parameter of fiber diameter that also produces a systematic change in the development of beads on the fibers. (C) 2007 Elsevier Ltd. All rights reserved.