Uniform arrays of Pt nanoparticles with controlled size and interparticle distance supported on glassy carbon electrodes were formed by a low temperature and low pressure Ar plasma using polystyrene-b-poly(2-vinylpyridine) diblock copolymers as templates. These particle arrays are ideal model catalysts for exploring particle size and interparticle spacing effects on the electrocatalytic activities of nanoparticles without the complications from particle aggregation and particle surface protecting molecular layers. Based on the rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) voltammetric results, it is found that both particle size and interparticle distance play important roles on the kinetic current density of O-2 reduction reaction (ORR) and the extent of H2O2 formation. Increasing particle size results in an increased kinetic current density of ORR, while increasing interparticle distance leads to decreased kinetic current density of ORR at high overpotentials. The RRDE results reveal that the extent of H2O2 formation increases with the decreased particle size and increased interparticle distance. The particle size effect is explained in terms of the particle size-dependent potential of zero total charge and the high mass transport rate during O-2 reduction. The interparticle distance effects are attributed to the interparticle distance dependent mass transport of H2O2. The study also points out the important role of the indirect oxygen reduction pathway in ORR on Pt. (c) 2012 Elsevier B.V. All rights reserved.