A polypropylene homopolymer was blended with ethylene-propylene rubber in different mixing ratios. The influence of the ethylene-propylene rubber content on the toughness behavior was investigated. According to the results of instrumented impact tests, brittle-to-tough transition temperatures were found for different ethylene-propylene rubber contents. Critical transition temperatures could be determined not only in the region of predominantly unstable crack propagation but also in the region of stable crack initiation. In situ measurements provided information on the deformation processes on the crack tip. (c) 2006 Wiley Periodicals, Inc.