The effects of incorporation of rubber particles on the environmental stress cracking (ESC) behaviour of rubber modified polystyrene (PS) and styrene-acrylonitrile copolymer (SAN) in Freon vapour have been investigated in view of the rubbery particle size and rubber content. Core-shell particles such as poly(n-butyl acrylate) rubber core/PS or poly(methyl methacrylate) shell particles of 0.2-2 mu m in rubbery core diameter were prepared, and these particles were incorporated into a PS or SAN matrix. The larger particles, about 1-2 mu m, showed the higher ESC resistance for rubber modified PS, whereas for SAN alloys small particles around 300 nm were the most effective in ESC resistance. Microscopic examination on the Freon-exposed specimen surface revealed that the surface damage is well developed craze in homo PS, and the development of craze is suppressed with increasing rubber content. The yielded lines were observed in homo SAN, and they became vague and diffuse with increasing rubber content. From these results, it was concluded that the role of rubber particles in the ESC process of rubber modified styrenic polymers in Freon vapour is to promote surface plasticization and to suppress the development of local damage on the specimen surface.