We describe the first example of redox catalysis using a dissolved electroactive nanoparticle, based on the oxidation of water by electrogenerated IrOx nanoparticles containing Ir-VI states, in pH 13 solutions of 1.6 +/- 0.6 nm (dia.) (IrOx)-O-IV nanoparticles capped solely by hydroxide. At potentials (ca. +0.45 V) higher than the mass transport-controlled plateau of the nanoparticle Ir-V/VI wave, rising large redox catalytic currents reflect electrochemical generation of Ir-VI states, which by +0.55 V and onward to +1.0 V are shown by rotated ring disk electrode experiments to lead with 100% current efficiency to the oxidation of water to O-2, O-2 production at +0.55 V corresponds to an overpotential eta of only 0.29 V, relative to thermodynamic expectations of the four electron H2O -> O-2 reaction. The Ir site turnover frequency (TO, mol O-2/Ir sites/s) is 8-11 s(-1). Controlled potential coulometry shows that all Ir sites in these nanoparticles (average 66 Ir each) are electroactive, meaning that the nanoparticles are small enough to allow the required electron and proton transport throughout. Both the overpotential and TO values are nearly the same as those observed previously for films electroflocculated from similar IrOx nanoparticles, providing the first comparison of electrocatatysis by nanoparticte films with redox catalysis by dissolved, diffusing nanoparticles.