Abstract The free radical chain-growth cross-linking copolymerisation of an unsaturated polyester resin with styrene is investigated in isothermal conditions using different techniques. By modulated temperature differential scanning calorimetry (MTDSC) the rate of polymerisation is observed simultaneously with the (partial) vitrification of the reacting material. An important autoacceleration closely before vitrification corresponds to an accelerating styrene consumption. The autoacceleration occurs long after the gelation of the material, which implies that termination reactions involving small radicals are still important at an advanced conversion. The deceleration of these termination reactions causes the autoacceleration, which is stopped when the propagation reaction slows down due to vitrification. MTDSC proves to be very suitable for studying the interrelations between vitrification and the polymerisation kinetics.