The behavior of the solution jet during electrospinning is a consequence of the balance between electrical, surface tension, and viscoelastic forces. Solution rheology, which is known to have a significant effect on the structure of the electrospun polymer, may also affect jet evolution and breakdown. In this study, the effect of polymer molecular weight and concentration on the behavior of polystyrene solution jet was investigated using high speed camera (2000 frames/s). The results indicate that the degree of elongational flow, bending instability, and jet branching depends on polymer molecular weight and concentration. It was observed that jet thinning and solidification may occur at different distances from the capillary when the rheological conditions are varied. (c) 2006 Elsevier B.V. All rights reserved.