The effects of two low-profile additives (LPA), namely poly(vinyl acetate) and poly(methyl methacrylate), on the rheokinetics and morphological changes up to gelation in the cure of unsaturated polyester (UP) resins at 110-degrees-C were investigated by using a Haake viscometer, a differential scanning calorimeter, a Fourier-transform infra-red spectrometer and a scanning electron microscope. Adding LPA to the neat UP resins could increase the gel time and the gel conversion to differing extents, depending on the types of LPA added. The experimental results were explained by a microgel-based gelation mechanism on the basis of the integrated rheology-reaction kinetics-morphology results and by ternary static phase characteristics for styrene/UP/LPA systems at 25-degrees-C.