A brief literature survey is presented on the models predicting the composition dependence of fracture properties of heterogeneous polymer systems. Some of the models cannot predict composition dependence properly or are limited to a narrow composition range, while more exact ones are difficult to use because of their complexity. A different approach is presented, which takes into consideration matrix characteristics, component interaction, changing stiffness and the decrease of matrix cross-section due to modification Particulate filled and elastomer modified polypropylene blends and composites were prepared in the 0-0.3 volume fraction composition range, and strain energy release rate as well as notched Izod impact strength were determined. The proposed model proved to be valid for the studied blends and composites and revealed that the major energy absorbing process is the plastic deformation of the matrix, which, in the studied case, is enhanced by the presence of the second component. Data taken from the literature for the most diverse systems from elastomer modified thermoplastics to particulate filled epoxy resins were analysed in the same way and the approach proved to be valid in most cases. A correlation was found between fracture characteristics and yield stress of the systems. The structure of composites is an often neglected factor; orientation of anisotropic particles as well as aggregation strongly influence both the absolute values and composition dependence of fracture resistance.