Effects of a monofunctional benzoxazine diluent (Ph-a) on properties of a bifunctional benzoxazine resin (BA-a) have been investigated. The BA-a/Ph-a mixtures are miscible in nature rendering the properties highly dependent on their compositions. The viscosity of the BA-a resin can be reduced to one third using only about 10% by weight the Ph-a diluent. The addition of the Ph-a resin into the BA-a resin can also lower the liquefying temperature of the resin mixtures whereas the gel point is marginally decreased. The gel point, which depends on the BA-a/Ph-a mixtures and the cure temperature, was determined by the frequency independence of loss tangent in the vicinity of the sol-gel transition. The relaxation exponent values of the copolymer were found to be 0.24-0.55, which is dependent on the cure temperature. Gel time of the BA-a/Ph-a systems decreases with increasing temperature according to an Arrhenius relation with activation energy of 60.6 +/-1.5 kJ/mol. Flexural moduli of the BA-a/Ph-a polymers also increase with the Ph-a mass fraction, however, with the sacrifice of their flexural strength and glass-transition temperature. (c) 2007 Wiley Periodicals, Inc.