Syndiotactic polystyrene (sPS) is a chemically resistant, high-heat, semicrystalline polymer which is currently under development by The Dow Chemical Co. The research reported herein was undertaken to determine the critical fracture strength, i.e., the critical stress intensity factor, K 1C , and the fracture energy, G 1C , of sPS. The studies were aimed at developing a basic understanding of the failure mechanism and toughness of sPS. This work included investigations of the effect of molecular weight, as well as flow-induced anisotropy. Scanning electron microscopy (SEM) was used to aid in the determination of the failure mechanism. During failure testing, it was observed that sPS fails with a slow, controlled crack growth and ruptures with an almost nondetectable amount of yielding, as based on a tensile dilatometry investigation and a plane strain, biaxial yield experiment. The proposed failure mechanism, based on the scanning electron micrographs, is one of constrained crazing, followed by void coalescence with the spherulite nucleators acting as stress concentrators in the system. The damage appears to be greatly confined, with little initial cold-drawing of the spherulites. Addition of a nucleator reduces the K 1C values somewhat, as added nucleation sites proliferate the sites for stress concentration across the sample.