A failure mode transition from opening to shear has been reported in shear impact experiments of commercial polycarbonate with increasing impact velocity (Ravi-Chandar, K., 1995. On the failure mode transitions in polycarbonate under dynamic mixed mode loading. Int. J. Solids and Structures 32 (6/7), 925–938; Rittel, D., Levin, R., Maigre, H., 1997. On dynamic crack initiation in polycarbonate under mixed-mode loading. Mech. Res. Comm. 24 (1), 57–64). Impact velocities ranged typically from 20 to 60 m/s and the experiments were carried out at room temperature. Similar transitions have been reported for metallic materials and shear band formation at high velocities has been related to crack-tip adiabatic heating and material softening. This paper investigates the role of crack-tip heating in the observed trans ition. Results are reported and discussed for high velocity (50–60 m/s) shear impact experiments of commercial polycarbonate whose temperature is varied in the range of −120°C to +70°C. Up to −40°C, fracture proceeds along an initial kink angle of typically 60°. At −25°C the kink angle is reduced to 40° and less. At room temperature and above, the specimens no longer fracture by separation. A 2–3 mm shear band is then clearly noticeable at the crack-tip. Scanning electron and optical fractographic examination of the specimens confirm the initial operation of a shear fracture mechanism at −25°C (Teq=0.59 Tg in [K]), in contrast with an opening fracture mechanism at lower temperatures (crazing). Assuming that shear failure results from local heating above the glass transition temperature (150°C), the crack-tip temperature elevation is estimated to be at least 175°C. Significant crack-tip temperature changes put limitations on the usual assumptions of isothermal dynamic crack initiation.