A heterophasic reactor grade polypropylene-ethylene copolymer (RAHECO(R)) was diluted with a propylene-ethylene random copolymer to get 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 similar to 20 degrees C, -10 degrees C, 0 degrees C, +10 degrees C and room temperature. The critical interparticle distance shifts linearly over the range of temperatures from about 0.4 mu m at -20 degrees C to 1.3 mu m at room temperature. The results were compared with measurements of conventional notched Charpy impact strength at room temperature. The transition from brittle to tough impact behaviour was correlated to a transition of micromechanical deformation mechanisms from cavitation bands to croids. Cavitation of the rubber particles was always the first step of deformation. The in situ HVEM investigations additionally show that the particle distribution should be as homogeneous as possible. (C) 1997 Elsevier Science Ltd.