The effects of reactive modification on the interfacial strength in high-impact polypropylene materials have been investigated using a model system and demonstrated in two specific examples on high flow materials for thin wall injection moulding and reinforced compounds for engineering applications. Using Borealis' proprietary modification technology, the modification degree was controlled by varying the free radical initiator and co-agent content, respectively. The influence of the modification degree on the formation of ethylene-propylene copolymer grafted to polypropylene, which is believed to be the main reason for the strengthened interface between the PP matrix and EPR particles, the phase morphology and material properties was investigated. Depending on practical application requirements, a well defined material performance such as toughness, flowability, surface stability, etc. could be adjusted.