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• TiO 2 -MoO x was prepared as a support for Ir catalyst toward OER. • Interaction between TiO 2 -MoO x and Ir promotes formation of OER-active Ir(III) species. • During the OER, the chemical Interaction between Mo and Ir keeps in Ir(III) species and suppresses dissolution of Ir. • Therefore, Ir/TiO 2 -MoO x exhibited improved OER activity and stability compared Ir black, Ir/C and Ir/TiO 2. • Ir/TiO 2 -MoO x improve performance and durability of the PEMWE single cell than Ir black. To accomplish the goals of cost reduction and durability improvement of polymer electrolyte water electrolysis (PEMWE), we report a titanium oxide and molybdenum oxide composite (TiO 2 -MoO x) as a support for the iridium (Ir) nanoparticle catalyst (Ir/TiO 2 -MoO x) toward oxygen evolution reaction (OER). Incorporation of Mo(V) and Mo(VI) into TiO 2 significantly enhances the electrical conductivity of TiO 2. During the synthesis of Ir nanoparticles on TiO 2 -MoO x , Mo(V) is oxidized to Mo(IV) with suppressing oxidation of Ir(III) to Ir(IV). Consequently, Ir/TiO 2 -MoO x forms higher fraction of the OER-active Ir(III) species on Ir surface enhancing OER activity of Ir catalyst. During OER, the chemical interaction between Mo and Ir keeps in Ir(III) and lessens the dissolution of Ir(III), improving the durability of Ir/TiO 2 -MoO x. Thus, Ir/TiO 2 -MoO x exhibits superior OER activity and durability. The single cell test results demonstrate that, as an anode catalyst, Ir/TiO 2 -MoO x can improve the performance and durability of the PEMWE cell than Ir black. [ABSTRACT FROM AUTHOR]