Moderate increases (similar to 50-75%) in the toughness of bismaleimides (BMIs) were achieved with very low-molecular-weight (similar to 1000 g/mol) imide thermoplastics at low levels of thermoplastic loading (similar to 10-20%). The thermoplastic was introduced into the BMI using a simple, one-pot, reactive solvent approach. In this approach, the reactive diluent of a two-part BMI was used as the reaction solvent for the thermoplastic synthesis. The BMI monomer was then dissolved in the thermoplastic reaction solution to yield a low-viscosity homogenous prepolymer. The viscosity of the thermoplastic solution was similar to 6 Pa S at 55 degrees C. The effect of thermoplastic loading and molecular weight on viscosity was determined by rheology, and the fracture toughness of neat resin plaques was determined by compact tension. Increasing the thermoplastic loading increased prepolymer viscosity without improving toughness, while increasing the thermoplastic molecular weight increased the toughness by only 25% more than the lowest-molecular-weight thermoplastic, yet increased viscosity fivefold. Fracture surfaces showed no obvious phase separation by scanning electron microscopy.