Both melt viscosity (eta(o)) and elasticity (correlated here with the longest melt relaxation time) were found to control the diameter distribution of meltblown fibers. Fibers were formed by melt blowing binary polystyrene (PS) blends containing widely differing component molecular weights using a custom-built laboratory apparatus. Varying the concentration and molecular weight of a high molecular weight PS provided independent control over eta(o) and lambda(1). These rheological parameters influence the average diameter (d(av)) and the distribution of diameters (coefficient of variation, CV) of meltblown fibers in different ways. Increasing eta(o) go leads to an increase in d(av) but has little impact on CV. On the other hand, increasing lambda(1) beyond a threshold value reduces CV while simultaneously increasing d(av). A one-dimensional slender-jet theoretical model with both upper convected Maxwell and Phan-Thien and Tanner constitutive equations was developed to investigate the influence of viscoelasticity and processing parameters on the properties of meltblown fibers. This model predicts a strong dependence of fiber diameter on the air shear stress and variations in fiber diameter with viscoelasticity that are in qualitative agreement with the experimental results. We believe these results suggest that carefully controlling the viscoelastic profile of polymers used in melt blowing is a viable approach for producing nanofibers with narrow fiber diameter distributions using current commercial equipment. Published by Elsevier B.V.