Electrocatalytic activity for the oxygen reduction reaction (ORR) of the carbon-supported Pt catalysts obtained by chemical and electrochemical deposition has been studied in 0.10 M HClO4 at 293 K in the expectation to gain new insights into the influence of the substructure of platinum on its catalytic properties for the ORR. Electron microscopy, XRD and electrochemical techniques have been employed to demonstrate that: (i) the 'chemically' synthesised Pt/C catalysts contain highly-dispersed isolated metal particles at intermediate metal loadings and both isolated crystallites and crystallite aggregates at higher loading; (ii) the electrodeposited Pt/C materials are 'nanostructured', i.e. contain large (up to ca 350 nm) aggregates with the deposition potential dependent morphology composed of comparatively small (<20 nm) platinum crystallites. No structural dependence of the ORR catalytic properties for the 'chemically' synthesised Pt/C catalysts has been revealed, except for the well-known negative size effect. On the contrary, platinum electrodeposited on glassy carbon or furnace black demonstrates atypically high ORR catalytic activity, especially, when electrodeposition is undertaken at 0.10 V. Presumably, easily oxidisable defect sites, intergrain boundaries in particular, present at comparatively high concentrations in the electrodeposited materials provide efficient adsorption of O-2, which further reduction occurs at the highly catalytically active Pt(111) facets. Structural defects present at an optimal concentration are concluded to promote the ORR catalytic activity of platinum, but excessive defectiveness of the Pt catalyst might hinder the kinetics of the oxygen electroreduction. (C) 2014 Elsevier B.V. All rights reserved.