Many chemical reactions are catalysed by metal complexes, and insight into their mechanisms is essential for the design of future catalysts. A variety of conventional spectroscopic techniques are available for the study of reaction mechanisms at the ensemble level, and, only recently, fluorescence microscopy techniques have been applied to monitor single chemical reactions carried out on crystal faces(1) and by enzymes(2-4). With scanning tunnelling microscopy ( STM) it has become possible to obtain, during chemical reactions, spatial information at the atomic level(5-9). The majority of these STM studies have been carried out under ultrahigh vacuum, far removed from conditions encountered in laboratory processes. Here we report the single-molecule imaging of oxidation catalysis by monitoring, with STM, individual manganese porphyrin catalysts, in real time, at a liquid-solid interface. It is found that the oxygen atoms from an O-2 molecule are bound to adjacent porphyrin catalysts on the surface before their incorporation into an alkene substrate.