environmental stress cracking (esc) measurements for various impact polystyrenes were performed using a constant load technique with the specimens in contact with a 50/50 solution of cotton seed oil and oleic acid. it was shown that esc in impact polystyrene is controlled by the transport of the aggressive liquid through a pre-established dry craze structure where capillary pressure is the driving force. at moderate stress levels just above the critical stress for environmental cracking, there is an apparent incubation time for the dry craze formation. the craze incubation time is strongly influenced by thermal stresses induced by the gel particles. as a consequence, esc is a two-stage process involving both an incubation time and actual crack growth. control of the craze structure to maximize fibril content is essential for good esc resistance. the craze fibril content can be altered by variables such as gel particle size, matrix molecular weight, plasticizer content, and rubber content