Electrochemical experiments with a rotating disk electrode are used to measure specific catalytic activity of Pt/C structures in the oxygen reduction reaction at the density of Pt nanoparticles on the glassy carbon support surface below one monolayer. The specific activity maximum is found at the coverage of about 0.4 monolayer. An explanation of the observed dependence is suggested that is based on consideration of the relationship between the surface density and charge state of the system of metallic catalyst particles. A numeric model is developed that describes charge transfer in the catalyst structure due to the difference in the work functions between the metal nanoparticles and support with account for the discrete nature of the nanoparticle charging and their mutual polarization. Calculations show that the carbon support coverage by Pt particles of about 0.4 monolayer corresponds to the largest amount of charged particles with the maximum energy of electrons, which provides the maximum catalyst activity and explains the dependence observed in the experiment.