Phase stability diagrams of Ti-M-O-C (M=Zr, Nb, and Ta, molar ratio of Ti and M=1:1) systems at 1800K were drawn as a function of the carbon activity, oxygen partial pressure, and solution formation characteristics. The solid solution characteristics were varied with the kind of M. The solid solution carbide, (Ti0.5Zr0.5)C, was less stable than the TiC-ZrC mixture while other solid solution carbides (Ti0.5Nb0.5)C and (Ti0.5Ta0.5)C were more stable than the mixtures of monocarbides. Thus, the (Ti0.5Zr0.5)C phase was not included in the phase stability diagram of the Ti-Zr-O-C system unlike the other solid solution carbides. The validity of the drawn stability diagrams was proved by experimental results. Thus, the conditions for synthesis of solid solution carbides by carbothermal reduction, or fabrication of TiC-based composites with solid solution phases, can be deduced using the phase stability diagrams.