Rubber-modified polystyrenes were prepared by bulk polymerization with seven different butadiene–styrene copolymers of differing chemical microstructures. The products were structurally characterized through measurement of the swell ratio, percent insolubles, intrinsic viscosity of the soluble fraction, and transmission electron microscope photomicrographs. Increasing initiator concentration or raising polymerization temperature gives lower molecular weight, higher rubber crosslink density, and decreased grafting. Increasing rubber content generally leads to aggregation. Tensile stress–strain curves and Izod impact strengths were measured. High Izod impact strength and increased elongation to break are favored by increasing matrix molecular weight, rubber content, and extent of grafting.