Platinum dendrimer-encapsulated nanoparticles (DENs) containing an average of 55, 100, 147, 200, and 240 atoms were prepared within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers. These DENs were immobilized on glassy carbon electrodes, and the effect of particle size on the kinetics of the oxygen reduction reaction (ORR) was quantitatively evaluated using rotating disk voltammetry. The total areas of the Pt DENs were determined by electrochemical CO stripping and hydrogen desorption, and the results were found to be in reasonable agreement with calculated values. The largest particles exhibited the highest specific activities for the ORR.