Metal oxide (MO x ) materials are effective catalysts or cocatalysts in a range of electrochemical reactions and energy systems. A key component of their redox chemistry is protoncoupled electron transfer (PCET). Reported here are studies of isolated cerium oxide nanoparticles deposited on fluorine-doped tin oxide (FTO) electrodes with a Langmuir−Blodgett trough and calcined. Cyclic voltammograms of these films showed welldefined, quasi-reversible waves. The E 1/2 values moved with pH by −64 ± 4 mV/pH, close to the ideal Nernstian −59 mV/pH for a 1e − /1H + couple. These results imply that the electroactive CeO− H bonds had an average bond dissociation free energy (BDFE) of 78.6 ± 1.5 kcal mol −1 . Integration of the faradaic currents indicates that 0.15 ± 0.04 electrons can be added per cerium atom in the nanoparticle film or ∼20% of the surface cerium atoms. This study shows how electrochemical investigations can elucidate the stoichiometry and thermochemistry of PCET processes of MO x nanoparticle films. Comparison of these results with those for related ceria nanoparticles shows the remarkable range of properties of this useful material.