AbstractThe performance of styrene‐acrylonitrile copolymers under alternating stresses has been examined. Information has been obtained concerning the influence of acrylonitrile content and of molecular weight on thermal effects due to hysteresis, on cyclic lifetime to craze initiation, on average cycles to fracture, and on fatigue fracture surface morphology. The influence of an elastomeric second phase has been determined by comparing fatigue performance under comparable conditions of both styrene‐acrylonitrile (SAN) and acrylonitrile‐butadiene‐styrene (ABS). The stress concentrating effects of the rubber particles cause earlier initiation of fatigue‐induced damage and lower fatigue fracture resistance. The fatigue‐induced specimen temperature rise is greater in ABS than SAN and it increases linearly with test frequency and as the square of the stress amplitude. The fracture surface morphology of ABS, which differs from that of unmodified SAN and also from that of rubber modified polystyrene, i