The role of fluid elasticity in the formation of fibers from polymer solution by electrospinning is investigated. Model solutions with different degrees of elasticity were prepared by blending small amounts of high molecular weight polyethylene oxide (PEO) with concentrated aqueous solutions of low molecular weight polyethylene glycol (PEG). The elastic properties of these solutions, such as extensional viscosity and the longest relaxation time, were measured using the capillary break-up extensional rheometer (CaBER). The formation of beads-on-string and uniform fiber morphologies during electrospinning was observed for a series of solutions having the same polymer concentration, surface tension, zero shear viscosity, and conductivity but different degrees of elasticity. A high degree of elasticity is observed to arrest the breakup of the jet into droplets by the Rayleigh instability and in some cases to suppress the instability alto-ether. We examine the susceptibility of the jet to the Rayleigh instability in two ways. First, a Deborah number, defined as the ratio of the fluid relaxation time to the instability growth time. is shown to correlate with the arrest of droplet breakup, giving rise to electrospinning rather than electrospraying. Second. a critical value of elastic stress in the jet, expressed as a function of jet radius and capillary number, is shown to indicate complete suppression of the Rayleigh instability and the transition from 'beads-on-string' to uniform fiber morphology. (c) 2006 Elsevier Ltd. All rights reserved.