Quantitative analysis of the differential of heat capacity with temperature, dC(p)'/dt, signal from modulated-temperature differential scanning calorimetry (M-TDSC) allows the extent of phase mixing to be calculated for interpenetrating polymer networks (IPNs). A new characterisation method for interpenetrating polymer networks is proposed in which the dC(p)'/dT signal was used to analyse phase structure and composition. It is believed that the dC(p)'/dT with temperature signal will become a useful tool and a powerful complement to solid-state NMR, scattering and direct nonradiative energy-transfer methods for analysing the morphology of IPNs.