The study of electrospinning polymer solution jets, and the evolution of the polymer entangled network during electrospinning is of interest for understanding of the microstructure of the resulting nanofibers. Fast X-ray phase-contrast imaging was applied to investigate the flow of the first 10 mm of a straight jet of electrospinning PEO and PMMA semidilute solutions. The jet radius, velocity, and absorbance were measured at high resolution (0.67 mu m/pixel) and at extremely short exposure time (< 0.5 mu s), under a wide range of electrospinning conditions and solution concentrations. The flow field, measured by tracing silica microbeads, revealed laminar flow with axial velocities that implied significant mass loss due to evaporation. X-ray absorption measurements provided evidence for substantial polymer concentration rise along the jet, particularly at the jet boundaries, evidence of rapid evaporation. Furthermore, at high strain rates, the polymer concentration rose at the jet center as the velocity increased along the jet, implying polymer network lateral contraction due to axial stretching. Our findings confirm the theoretically predicted counteracting effects of evaporation and stretching on the polymer matrix within the jet.