The reaction kinetics of polyurethane and vinyl ester resin interpenetrating polymer network (PU/VER SIN) during reaction injection moulding (RIM) polymerization has been on-line monitored by using a Fourier transform infrared (FTi.r.) spectrometer linked up with a portable mini-RIM machine. For using VER, in which the pendent hydroxyl groups were capped with acetyl groups to minimize the possibility of chemical binding between the two networks, the two pairs of reactants interfere with each other during the course of synthesis. The unusual polymerization kinetics behaviour observed could be correlated well with the change of morphology of the system. For using VER containing pendent hydroxyl groups, the occurrence of chemical binding between the two networks could greatly affect the reaction kinetics and morphology of the system.