Phase　stability　of　(Ru1-x,Tix)O2　solid　solution　was　investigated　via　materials　computation　and　X-ray　diffraction　phase　analyses.　The　unit　cell　volume　of　(Ru1-x,Tix)O2　was　found　not　increasing　monotonously　with　the　fraction　of　TiO2,　making　the　volume-composition　relationship　no　longer　suitable　for　the　quantitative　phase　analysis.　A　monotonous　nonlinear　c/a-x　relation　obtained　from　the　ab　initio　density　functional　theory　(DFT)　computation　was　proved　critical　to　achieving　accurate　phase　analysis　results.　Gibbs　free　energy　of　mixing　and　its　derivatives　were　determined　for　(Ru1-x,Tix)O2　by　a　combination　of　DFT　and　thermodynamic　calculations,　where　approximation　of　the　temperature　dependence　of　the　interaction　parameter　(Ω)　played　a　critical　role.　It　was　found　that　the　conventional　approximation　method　of　arbitrarily　choosing　a　value　&　[10,25]　kJ　(mole　of　atoms)-1　for　Ω　near　a　critical　temperature　of　a　solid　solution　structure　failed　to　predict　the　correct　symmetry　and　span　of　the　miscibility　gap　for　(Ru1-x,Tix)O2.　In　comparison,　Ω　calculated　from　disordered　(Ru1-x,Tix)O2　was　found　a　good　approximation　at　the　critical　temperature.　The　calculated　miscibility　gap　for　(Ru1-x,Tix)O2,　therefore,　showed　good　agreement　with　the　accurate　quantitative　phase　analysis　results　for　the　samples　prepared　by　a　sol-gel　method.　©　2015　The　American　Ceramic　Society.