Understanding the effects of particle size and surface structure on the electrochemical behavior of nanomaterials is of critical importance to the optimization of electrocatalysts. In this work the electrocatalytic activity of arrays of spherical gold nanoparticles (AuNPs) of varying diameter was imaged using scanning electrochemical microscopy (SECM), focusing on the oxygen reduction reaction (ORR) and the electrooxidation of hydrogen peroxide. Unlike most previous reports of catalyst array screening wherein tip generation-substrate collection (TG-SC) is the favored approach, redox competition and tip collection modes were instead employed. For both reactions studied the electrocatalytic activity, normalized to surface area, was observed to increase with decreasing particle diameter in the range 5-50 nm. Characterization of the AuNP surface structure using lead underpotential deposition revealed that this trend can be correlated to the ratio of Au(1 1 0) to Au(1 1 1) sites. For the ORR this is consistent with the established view that Au(1 1 0) is more active than Au(1 1 1). Conversely, for hydrogen peroxide electrooxidation on AuNPs this is in contrast to Au single crystal data, suggesting the subtle influence of higher index sites on this reaction. - Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.