The fracture behavior of three high-impact polystyrene (HIPS) materials has been investigated using constant load at 1/2 h interval tests of margarine-exposed compact tension fracture mechanics specimens. Scanning electron microscopy has also been done on the fracture surfaces to correlate the micromechanics of fracture to the fracture test results. The results indicate that the proposed tests provide a useful approach to study the environmental fracture resistance (EFR) of HIPS under the influence of an aggressive environment. It was found that the EFR of HIPS under margarine, with comparable rubber content, improves with decreasing rubber particle size and increasing molecular weight.