The thermal stability of refractory carbide/boride composites in oxidizing environment was analyzed. The multi-component thermodynamic stability diagrams for the complex (Hf, Ta, Zr, Si)-C(B)-O composite systems were generated using the proposed linear inequality method. In this method, the stability area of compound is the solution of a set of linear inequalities which is directly obtained from the free energy changes of general chemical reactions. The generated thermodynamic stability diagrams were used to analyze the formation of multilayer oxide scale in HfC-TaC and ZrB2-SiC composites, and correlated with experimental results. (C) 2002 Elsevier Science B.V. All rights reserved.