PVA nanofibers electrospun from solutions with relatively low polymer concentrations (below 8 wt%) tend to be no longer smooth, but display barbs which occur regularly spaced along the nanofiber length. Such structures are of interest for a number of technical applications, since they affect the fiber packing, pore sizes and the internal surfaces in the nanofiber nonwovens. This paper reports both, experimental and theoretical results, allowing to elucidate the mechanism responsible for barb formation. It is found that barb formation can be explained theoretically in terms of a relatively slow charge relaxation within the jet compared to the development of the secondary electrically driven instabilities which locally deform the jet surface. Both, the electric conductivity of polymer solutions and their viscoelasticity, are key parameters controlling the competition between charge relaxation and rate of growth of capillary and electrically driven, secondary, localized perturbations of the jet surface and thus barb formation. In this paper a nonlinear theoretical model is proposed that is able to mimic the main morphological trends recorded in the experiments. (c) 2010 Published by Elsevier Ltd.