This study examined the change of toughness with crack velocity of two different types of acrylonitrile-butadiene-styrene (ABS) copolymers. Small laboratory specimens were used which could be precisely loaded and then allowed to stress relax before crack initiation. An experimental technique was employed which provided for good isolation of the crack initiation and propagation processes. Additionally, using this technique, it was possible to study the toughness of the materials for crack velocities from the threshold condition to the limiting crack velocity condition.