Phase　diagram　of　iridium-tin　oxide　(Ir-Sn-O)　was　calculated　by　a　combination　of　ab　initio　density　functional　theory　(DFT)　and　thermodynamic　calculations.　Results　suggested　that　the　phase　separation　that　had　been　reported　in　literature　for　Ir-Sn-O　was　through　a　typical　spinodal　decomposition　mechanism.　Ir0.534Sn0.466O2　was　prepared　by　a　thermal　co-decomposition　method　at　320　and　450°C.　Quantitative　phase　analyze　based　on　DFT,　X-ray　diffraction　and　high-resolution　transmission　electron　microscopy　confirmed　the　spinodal　nature　of　the　phase　separation.　The　present　fundamental　study　is　an　important　reference　for　phase　and　microstructure　design　of　Ir-Sn-O　for　various　electrocatalysis　applications.　The　DFT-assisted　quantitative　phase　analysis　has　provided　peer　researchers　with　a　novel　reliable　phase　analysis　approach　for　complex　compounds.　©　2013　Elsevier　Ltd.