The　phase　diagram　of　Ru-Ce-O　was　calculated　by　a　combination　of　ab　initio　density　functional　theory　and　thermodynamic　calculations.　The　phase　diagram　indicates　that　the　solubility　between　ruthenium　oxide　and　cerium　oxide　is　very　low　at　temperatures　below　1100　K.　Solid　solution　phases,　if　existing　under　normal　experimental　conditions,　are　metastable　and　subject　to　a　quasi-spinodal　decomposition　to　form　a　mixture　of　a　Ru-rich　rutile　oxide　phase　and　a　Ce-rich　fluorite　oxide　phase.　To　study　the　spinodal　decomposition　of　Ru-Ce-O,　Ru0.6Ce0.4O2　samples　were　prepared　at　280　degrees　C　and　450　degrees　C.　XRD　and　in　situ　TEM　characterization　provide　proof　of　the　quasi-spinodal　decomposition　of　Ru0.6Ce0.4O2.　The　present　study　provides　a　fundamental　reference　for　the　phase　design　of　the　Ru-Ce-O　electro-catalyst.