Environmental stress cracking behaviour of polystyrene and styrene-acrylonitrile copolymer (SAN) has been studied in Freon vapour in view of molecular weight and acrylonitrile content. The critical stress for craze or crack initiation was measured using three-point bending test and the surface damage was examined by scanning electron microscopy. The critical stress increased with an increase of acrylonitrile content in SAN copolymer, which suggests that the craze initiation stress has a good correlation with the solubility parameter difference between the polymer and crazing agent. However, molecular weight did not have any significant effects on the critical stress for craze initiation. For homopolystyrene, relatively large-sized craze was developed in Freon vapour due to the increase of softened layer thickness in the developed craze wall. As acrylonitrile content increases the development of craze is suppressed.