Isothermal kinetic results were obtained for reactions between aromatic diamines with variable steric hindrance and two bismaleimide monomers, one with a structure hindered oc from the two maleimide functions (BMEt) and the other not (BM). In the temperature range 120-200 degrees C it was found that these two reactions competed in the mixtures. It was thus possible to establish an order of molten-state reactivity of the diamines as a function of the steric hindrance of alkyl groups on NH2 functions in binary systems with monomer BM. At 173 degrees C, increasing hindrance on the diamine reduced the extent of the addition reaction and favoured the homopolymerization reaction of BM in the molten state. The same result was obtained in the hindered BMI-non-hindered diamine system. Because of the coexistence of steric hindrance of alkylated substituents on maleimide double bonds and steric hindrance of the diamine on reactive amine functions, some molten-state reactions are largely disfavoured.