Iridium oxide (IrO x ) is one of the most efficient electrocatalysts for water oxidation reaction (WOR). Here, WOR electrocatalysis by 1.6 nm IrO x nanoparticles (NPs) electrophoretically deposited onto spectroscopic graphite (Gr) and basal plane highly ordered pyrolytic graphite (HOPG) was studied as a function of NPs' capping ligands and electrodeposition substrate. On Gr, OH Àand H 2 O-capped NPs exhibited close sub-monolayer surface coverages and specific electrocatalytic activity of 18.9-23.5 mA nmol À 1 of Ir IV/V sites, at 1 V and pH 7. On HOPG, OH Àcapped NPs produced films with a diminished WOR activity of 5.17 � 2.40 mA nmol À 1 . Electro-wettability-induced changes impeded electrophoretic deposition of H 2 O-capped NPs on HOPG, WOR currents being 25-fold lower than observed for OH Àcapped ones. The electrocatalysis efficiency correlated with hydrophilic properties of the substrate electrodes, affecting morphological and as a result catalytic properties of the formed IrO x films. These results, important both for studied and related carbon nanomaterials systems, allow fine-tuning of electrocatalysis by a proper choice of the substrate electrode.