Catalyst effects on cure reactions of a bismaleimide [4,4'-bismaleimidodiphenylmethane (BMI)] associated with a liquid aromatic dicyanate ester [1,1'-bis(4-cyanatophenyl)ethane (BEDCy)] and with a powder type of aromatic dicyanate ester [bisphenol A dicyanate (BADCy)] were thoroughly investigated by in situ FTIR and DSC dynamic scanning. In noncatalyzed blend systems, coreactions between the dicyanate ester and bismaleimide always occur,. and thus the formation of the pyrimidine and/or pyridine structures occurs. The pyrimidine structure always predominates. The use of a dicyanate-sensitive catalyst facilitates the formation of a sequential interpenetrating network (IPN). The extent of the sequential IPN depends on the level of catalyst and the type of matrix materials, and thus the extent of coreactions. Probable reaction paths were also proposed for various formulations of hybrid blends. (C) 2004 Wiley Periodicals, Inc.