The phase separation process and morphology evolution of 5 wt% polysulfone (PSF) nanofibrous membranes toughened epoxy resin at different temperatures were investigated by synchrotron radiation s. angel X-ray scattering (SR-SAXS), phase contrast microscopy (PCM) and scanning electron microscopic (SEM). The onset time of phase separation obtained by different methods was basically identical. As curing proceeded at constant temperature, the scattering peak corresponding to the maximum scattered intensity shifted to a smaller scattering vector (q(m)), and the average diameters of PSF spheres increased which showed a phase separation pattern of nucleation and growth mechanism. PSF spheres exhibit random alignment in "sea-island" morphology, which was attributed to the in situ phase separation PSF nanofibers. Also, the effects of phase separation kinetics on the phase morphology and fracture toughness of nanofibrous membranes toughened epoxy resin were investigated. Results showed that the phase separation process was faster than the curing reaction process, which implied that the diffusion coefficient of PSF in epoxy resin increased with increasing the curing temperature, resulting in the increased PSF sphere size that in turn improved the fracture toughness at higher temperature. (C) 2009 Elsevier B.V. All rights reserved