As an electrocatalyst in a monolithic PV-electrolytic cell for water splitting hydrogen generation Co3O4 films were prepared by a paste coating method using Co3O4 particles. Different sized Co3O4 particles with average diameters of 145.9, 63.3 and 36.5 nm were prepared using a supercritical hydrothermal synthesis method. Electrochemical properties with respect to the particle size in the film were investigated by evaluating overpotential, charge transfer resistance (R-ct), and number of active sites (q*). The relation between overpotential in water oxidation at 5 mA/cm(2) and BET surface area showed a slope of -73.8 +/- 6.6, implying strong particle size dependence on electrocatalytic activity. Moreover, the R-ct and q* values and the actual hydrogen evolution rate indicated that the electrocatalytic activity of Co3O4 is attributed to physical properties (e.g. particle size) of the film. Whereas, intrinsic single site activity of the film was not significantly changed with respect to the particle size in the film. Copyright (C) 2011, Hydrogen Energy Publications, LLc. Published by Elsevier Ltd. All rights reserved.