This study deals with the morphological and chemical-state changes caused by the degradation of nanocomposite electrocatalysts - fabricated by pulsed potentiostatic co-electrodeposition and subsequently pyrolysed - under oxygen reduction reaction (ORR) conditions in aqueous alkaline solution. Variations in shape, dimensions and chemical state of the Mn-centres were followed by quasi-in situ synchrotron-based scanning photoelectron microscopy with submicron lateral resolution, combined with ex situ Raman measurements, in correspondence of different cyclovoltammetric ageing stages. The decline of the electrocatalytic performance is accompanied by size variations of the MnOx particles that are initially similar to 30 nm in diameter, then shrink to similar to 10 nm and subsequently grow to similar to 45 nm after prolonged ORR. Concerning chemical state, the pristine Mn-0,Mn-II nanoparticles are converted to Mn-III,Mn-IV oxy-hydroxides as a result of a dissolution/redeposition process favoured by the oxygen environment. (C) 2015 Elsevier B.V. All rights reserved.