Two heterophasic reactor-grade propylene-ethylene copolymers (RAHEC(R)) were diluted with propylene-ethylene random copolymers to obtain materials with constant EPR/PE-particle diameter but various interparticle distances. According to the results of instrumented impact tests, brittle-to-tough transitions were found at different temperatures. The critical interparticle distances shift linearly over the observed range of temperature. Critical interparticle distances could be determined not only in the region of predominantly unstable crack growth but also in the region of predominantly stable crack growth.