Both the isothermal and non-isothermal polymerizations of N,N'-bismaleimide-4,4'-diphenylmethane (BMI) with barbituric acid (BTA) were investigated by the differential scanning calorimeter. The experimental results showed that the polymerizations of BMI with BTA were governed by the competitive Michael addition reaction and free radical polymerization mechanisms. Furthermore, the contribution of free radical polymerization becomes more important when the mole fraction of BTA decreases. H-1 NMR and C-13 NMR measurements further support the coexistence of the Michael addition reaction and free radical polymerization mechanisms. A preliminary kinetic model that took into account the competitive Michael addition reaction and free radical polymerization mechanisms was developed. POLYM. ENG. SCI., 51: 1188-1197, 2011. (C) 2011 Society of Plastics Engineers