A　phase　stability　diagram　of　ruthenium-zirconium　oxide　(Ru-Sn-O)　was　constructed　by　a　combination　of　ab　initio　density　functional　theory　and　thermodynamic　calculations.　Results　suggest　that　the　phase　separation/segregation　that　has　been　reported　in　the　literature　for　the　RuO2-SnO2　system　is　through　a　typical　spinodal　decomposition　mechanism.　Ru0.45Sn0.55O2　films　were　prepared　by　thermal　co-decomposition　of　precursors　at　500　degrees　C　for　varied　duration.　Quantitative　phase　analyses　of　the　prepared　films　based　on　X-ray　diffraction　and　high-resolution　transmission　electron　microscopy　confirmed　the　spinodal　nature　of　the　phase　separation.　The　present　fundamental　study　provides　a　theoretical　guideline　for　the　phase　and　microstructure　design　of　Ru-Sn-O　based　mixed　oxides　for　electrocatalysis　applications.