The polymerisation kinetics of 1,1'-(methylenedi-1,4-phenylene)bismaleimide (MDP-BMI) with the aromatic diamine 1,1'-diaminodiphenylmethane (DDM) has been investigated. Fourier-transform near infrared (FT-NIR) spectroscopy was used to obtain quantitative data for the various functional groups in BMI/DDM mixtures with respect to time during isothermal cure over the temperature range 160-180 degrees C. Quantitative rate data were obtained for all the functional groups in the resins by mathematical differentiation of the time dependence of the concentrations. A mechanism for the polymerisation has been deduced from this rate data, in which the rate-determining step was found to involve a reaction between the primary or secondary amine and a hydrogen bonded complex formed between the maleimide and amine groups. The rare coefficients for the reaction of the amines with the complex were found to be four times larger for the primary amine than that for the secondary amine. The activation energy for the cure over this temperature range was found to be 43 kJ mol(-1).