A commercially available epoxy resin (E907) formulated with a viscosity-reducing styrene monomer and several additives was subjected to thermal cure studies and mechanical property measurements. Thermoplastic poly(arylene ether sulfone) (PES and poly(arylene ether phosphine oxide) (PEPO) with reactive amine or hydroxyl end groups were utilized to toughen and co-cure with the system. The cure cycle was optimized and the networks were analyzed via differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analyzer, scanning electron microscopy, sol-gel extractions, and fracture toughness. A model epoxy resin was prepared from a tetrafunctional epoxy, e.g., MY722, difunctional EPON828, styrene monomer, and benzoyl peroxide initiator (BPO), and was evaluated as a control to assess the possible role of the styrene monomer. The optimized cure cycle for E907 was 6 h at 93 degreesC, followed by a postcure of 2 h at 204 degreesC. The fracture toughness of E907 was increased only marginally with PES and PEPO. In contrast, the model epoxy resin demonstrated a positive effect due to the styrene monomer and BPO and exhibited significantly increased fracture toughness with PES modification.