The random strength of heterogeneous materials is considered as a function of volume fraction of the material components. A general macromechanical probabilistic model is created that permits one to consider all experimental results jointly, and therefore, minimize the experimental programme. A probabilistic description and corresponding method of statistical treatment, taking into account variability of the volume fraction on both the average strength and strength scatter, is offered. Experimental confirmation of the approach is presented using three examples of polymer blends of polyethylene and polypropylene. The influence of volume fraction scatter is analysed in detail, and it is noted that the scatter may lead to a significant increase in variability of the strength. A generalization of the approach is considered for other relevant problems, such as random thermal expansion as a function of temperature and random strength of multi-component (hybrid) materials.