The crazing behavior of blends of polystyrene (PS) and a low molecular weight polybutadiene (PB)was examined as part of a continuing study of toughening mechanisms in thermoplastic polymers. These PS/PB blends attain high levels of toughness from the stress-activated plasticizing action of the polybutadiene (PB), a mechanism that is active only in the region of a growing craze. The plasticization is therefore localized and leads to enhanced toughness without loss in stiffness. The net result of this plasticization is a reduction in craze flow stress accompanied by an increase in craze velocity, which, in turn, allows the specimen to reach substantial strains-to-fracture in uniaxial deformation under an imposed strain rate. The ability of the PB to plasticize a growing craze is expected to be a function of the mobility of the PB. To investigate the role of diluent mobility, tensile tests and craze velocity measurements were conducted at -20-degrees-C and compared with previously published results collected at 23-degrees-C. Although the blends displayed high levels of toughness at 23-degrees-C, the blends tested at -20-degrees-C showed brittle behavior. Craze velocities measured at -20-degrees-C were 2 orders of magnitude lower than the results at 23-degrees-C. Addition of 3 wt % PB at -20-degrees-C led to craze velocities only as large as those found in pure PS at 23-degrees-C. Comparison of the craze velocities with an asymptotic theoretical model describing the dependence of the craze velocity on the PB content showed good agreement with the results at -20-degrees-C and only fair agreement with the room-temperature results.