For the cure process of tetrafunctional epoxy resin/polysulfone (EP/PSF) blends, we investigated the effect of cure temperature and blend composition on the phase separation behavior by light scattering and the structure development during cure by an optical microscope. The EP/PSF blend without the curing agent was shown to exhibit an LCST-type phase behavior (LCST = 241 degrees C). At the early stage of curing, the EP/PSF blend was homogeneous at the cure temperature. As the cure reaction proceeded, the blend was thrust into a two-phase regime by the LCST depression caused by the increase in a molecular weight of the epoxy-rich phase, and the phase separation took place via a spinodal decomposition( SD) or nucleation and growth (NG) mode, depending on the blend composition and the cure temperature. When cured isothermally at 220 degrees C, the blend exhibited a sea-island morphology formed via the NG mode below 5 wt % PSF content, while the SD mode prevailed above 20 wt % PSF content. At the intermediate composition range, combined morphology with both sea-island and cocontinuous structure was observed. On the other hand, by lowering the cure temperature and/or increasing the content of PSF component, a two-phase structure with a shorter periodic distance was obtained. It seems that the rate of the phase separation is considerable reduced, while that of the cure reaction is not as much.