Abstract: Heterophasic polyolefin alloys, with a semi-interpenetrating network structure and an outstanding combination of high stiffness, low temperature toughness and optionally a low coefficient of thermal expansion of propylene polymers and elastomers, are produced by melt compounding propylene polymers with a co-continuous blend, which contains 5 to 60% by weight of gel and was produced by reacting mixtures of propylene polymers and elastomeric polyolefins or styrene butadiene ethylene terpolymers with ethylenically unsaturated, hydrolyzable organosilane compounds in the presence of silanol condensation catalysts. Heterophasic polyolefin alloys are suitable for the production of films, sheets, fibers, panels, coatings, injection molded article, pipes, hollow objects and foams.