The curing reaction of the unsaturated polyester/styrene system was investigated by differential scanning calorimetry (d.s.c.). The variation of the free-volume fraction (V-f) during curing was calculated from d.s.c. data by an empirical equation developed in a previous work, along with Simha and Boyer’s free-volume equations. Time-temperature shifts of V-f vs. ln(cure time) data at different curing temperatures showed that there is a critical point 1* after which the free-volume fraction profile formed a master curve with respect to an arbitrary reference temperature and yielded a single Arrhenius activation energy. This result indicated that polymerization after critical point 1* was V-f-dependent only. The experimental data showed that the conversion (alpha(cr1*)) at critical point 1* was higher than gel conversion (alpha(cr1)). The gel point (critical point 1) was related to the onset of the overlap of polymer coils. The critical point 1* was related to the onset of the formation of a macro-network without the presence of primary polyester chains, and termination was totally controlled by local segmental diffusion and not by propagating polymer chain selfdiffusion. The experimental results also revealed that both alpha(cr1) and alpha(cr1*) increased with increasing curing temperature. This behaviour was consistent with Turner’s critical concentration model for free-radical polymerization.