The linear thermal expansion of thirteen tungsten carbide cermets with cobalt binder was investigated experimentally over the temperature range from 68 to 1800 F. Cobalt contents varied from 2.5 to 60 per cent. Several compositions included additions of mixed carbides of titanium, tantalum, and columbium. The experimentally observed coefficients of thermal expansion for the various compositions were compared with coefficients analytically computed from the coefficients for the constituents. Three such analytical methods were evaluated. In one method, the coefficient of expansion of the mixture was computed by volume fractions and in a second method by weight fractions. In the third method, the computation accounted for the stresses set up in the mixture by the difference in thermal expansion of the carbide skeleton and the cobalt binder. The expansions of all these cermets agreed with the values computed by weight fractions or by the stress method within 12 per cent, and by volume fractions within 28 per cent. The cermets containing less than one per cent mixed carbides agreed with the expansion computed either by weight fractions or by stress within 8 per cent, the cermets containing more than five per cent mixed carbides agreed with values computed by volume fractions …