Oxygen evolution catalysed by calcium manganese and manganese-only oxides was studied in 18O-enriched water. Using membrane-inlet mass spectrometry, we monitored the formation of the different O2 isotopologues 16O2, 16O18O and 18O2 in such reactions simultaneously with good time resolution. From the analysis of the data, we conclude that entirely different pathways of dioxygen formation catalysis exist for reactions involving hydrogen peroxide (H2O2), hydrogen persulfate (HSO5−) or single-electron oxidants such as CeIV and [RuIII(bipy)3]3+. Like the studied oxide catalysts, the active sites of manganese catalase and the oxygen-evolving complex (OEC) of photosystem II (PSII) consist of μ-oxido manganese or μ-oxido calcium manganese sites. The studied processes show very similar 18O-labelling behaviour to the natural enzymes and are therefore interesting model systems for in vivo oxygen formation by manganese metalloenzymes such as PSII.