Nonreactive bisphenol A-based poly(arylene ether triphenyl phosphine oxide/diphenyl sulfone) statistical copolymers and a poly(arylene ether triphenyl phosphine oxide) homopolymer, each having a number-average molecular weight of about 20 kg/mol, were synthesized and solution-blended with a commercial dimethacrylate vinyl ester resin. Free-radical cured systems produced morphologies that were a function of both the amount of phosphonyl groups and the weight percentage of the copolymers. For example, highly hydrogen-bonded poly(arylene ether phenyl phosphine oxide) homopolymer/vinyl ester resin mixtures were homogeneous in all proportions both before and after the formation of networks. Copolymers containing low amounts (less than or equal to 30 mol %) of the phosphonyl groups displayed phase separation either before or during cure. The phase-separated cured materials generally had phase-inverted morphologies, such as a continuous thermoplastic copolymer phase and a particulate, discontinuous vinyl ester network phase, except for systems containing a very low copolymer content. The resin modified with a copolymer containing 30 mol % phosphine oxide comonomer showed improved fracture toughness, suggesting the importance of both phase separation and good adhesion between the thermoplastic polymer and the crosslinked vinyl ester filler phase. The results suggested that the copolymers with high amounts of phosphine oxide should be good candidates for interphase sizing materials between a vinyl ester matrix and high-modulus carbon fibers for advanced composite systems. Copolymers with low amounts of phosphonyl groups can produce tough, vinyl ester-reinforced plastics. The char yield increases with the concentration of bisphenol A poly(arylene ether phosphine oxide) content, suggesting enhanced fire resistance. The incorporation of thermoplastic copolymers sustains a high glass-transition temperature but does not significantly affect the thermal degradation onset temperature.