A model of the temperature response for reaction injection molding (RIM) operation is developed that assumes uniform mold temperature and a very fast reaction with slow heat transfer from the reacting mass to the mold and negligible heat losses to the surroundings. The model parameters are fit to data from a number of RIM experiments that agree well with published values of the physical constants for the reactants. The simple model can be fitted on-line to data from the RIM apparatus and the results used to detect and correct deviations of the part properties from its desired values which could otherwise result in the production of many defective parts.