This is the third in a five-part series describing the preparation of tough, high-performance thermosets from low viscosity, autoclave-processable prepolymers. The first 2 articles described toughening of bismaleimides (BMI) and epoxy with Linear imide thermoplastics of similar to 1000 g/mol. Highly processable prepolymers were obtained, which resulted in increases in fracture toughness for BMI of similar to 75-100%, while the fracture toughness of epoxy was increased by up to 220%. This article describes the preparation of a low-molecular-weight comb-shaped imide oligomer (similar to 4100 g/mol) and the effect of the oligomer architecture and end-group on BMI and epoxy prepolymer viscosity and fracture toughness. When an unreactive comb-shaped oligomer was incorporated in a BMI prepolymer (10% thermoplastic loading in the thermoset), the fracture toughness increased by 67% over that of an untoughed control, while a reactive oligomer increased the fracture toughness by 150% over an untoughened control. At 55 degrees C, the viscosity of the solution of the reactive comb-shaped imide in B was only 6.2 Pa . S. When the oligomer was dissolved in epoxy resin, the viscosity was less than 0.2 Pa . S at 90 degrees C, and the fracture toughness increased by 110 and 133% (at similar to 13% loading in the thermoset), relative to an untoughened control, depending on the reactivity of the end group. The T-g and high-temperature modulus of BMI and epoxy remained approximately the same relative to the untoughened controls.