The pH-dependent solution voltammetry (pH 1–13) of phosphate-stabilized, small (2 nm diameter) iridium oxide nanoparticles (IrIVO x NPs) is described and compared with that of (flocculated) films of the same size nanoparticles on electrodes. The IrIVO x NPs show waves with one electron/one proton formal potential dependency for the IrV/IV redox transformation and (below pH 6) for the IrIV/III reaction. Above pH 6, the IrIV/III reaction becomes a one-electron/two-proton process, unlike the one-electron/one-proton reactivity of the nanoparticles in films. The change is associated with surface oxide acid–base sites having pK A = 6 for solution phase nanoparticles that apparently are inactivated by the flocculation chemistry. Spectral isosbestic points are observed over the pH range of 5–8 for the dissolved nanoparticles. Controlled potential coulometry demonstrates that all of the Ir sites, throughout the nanoparticle, undergo the IrIV/III redox transformation. Whereas the IrIVO x NPs are stabilized at different pH values by phosphate ligation, the associated equilibria are somewhat sluggish, as indicated by small spectral differences for equi-pH nanoparticle solutions mixed with phosphate by different procedures. The IrIVO x NPs can also be capped with hydrophobic carboxylic acids, which allows extraction into nonpolar solvents such as CH2Cl2.