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 ~ 30 nm in diameter, then shrink to ~ 10 nm and subsequently grow to ~ 45 nm after prolonged ORR. Concerning chemical state, the pristine Mn0,II nanoparticles are converted to MnIII,IV oxy-hydroxides …