Epoxy molding compounds (EMCs) have become an integral component for high power electronics to work under harsh environment nowadays. For high temperature operation, polymer networks with high glass transition temperature (T-g) are required to ensure device stability. In this work, high T-g polymers were designed via controlled incorporation of in situ formed and thermally stable triazine structures into epoxy matrix. Reactions involved in the copolymer system of cyanate ester and epoxy (CE/EP) were investigated. Increasing ratio of cyanate ester dramatically promoted T-g of the copolymer up to 275 degrees C with improved heat resistance. High temperature aging and moisture absorption tests revealed that hydrolysis of polycyanurate network and rearrangement of cyanurate occurred during aging, especially for copolymers with higher than 75% of cyanate ester. Based on thermal properties and aging performance, the composition of CE/EP system was optimized. The formulated bisphenol A cyanate ester and biphenyl epoxy copolymer system has great potential to be applied as high temperature encapsulation materials in electronic packaging. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1337-1345