AbstractThe structure of crazes in plasticized polystyrene has been studied by means of small‐angle x‐ray scattering and optical interference microscopy. Addition of plasticizer causes a rapid increase in the mean fibril diameterDand a slow decrease in the craze fibril volume fractionvf. The crazing stress σcwas also measured and it was found that the productDσcis independent of plasticizer concentration. These results are shown to be consistent with the entanglement model for controllingvfand the meniscus instability model of craze thickness g