A model of heterogeneous combustion of carbonaceous composite materials (polymer-matrix composites, carbon/carbon, etc.) in supersonic hot gas flows is developed. The model describes four main physicochemical processes occurring at heterogeneous combustion: chemical reactions (mainly oxidizing), sublimation, thermomechanical erosion of a composite under the action of an external overrunning gas flow, and erosion due to an internal flow of gases generated by thermodecomposition of the material. Numerical simulation and comparison with experimental data for different types of composite materials showed that a contribution of the thermomechanical erosion into the whole process of heterogeneous combustion is rather considerable, and under certain conditions it can reach 60% of the total linear rate of combustion. It was established that, under actual conditions of heterogeneous combustion of composites in high-speed flows, processes of oxidizing and thermomechanical erosion prove to be coupled.