The classical view of a chemical reaction involves the formation and breaking of bonds, facilitated by the transfer of electrons between atoms. To fundamentally understand the mechanisms occurring, atomic resolution of the catalysts enabling the reaction is required. Using the recently developed York JEOL Nanocentre Environmental Scanning Transmission Electron Microscope, single atoms and nanoparticles can be observed in representative industrial reaction conditions, allowing for atomic scale quantification of catalyst deactivation mechanisms, such as Ostwald Ripening. This is initially understood through the industrially important process of methanol synthesis. Copper nanoparticles, one component of a methanol synthesis catalyst, are imaged under both Hydrogen and high vacuum conditions. This is the first use of ESTEM to study sintering, which is shown to be governed by the Ostwald Ripening mechanism and significantly enhanced by the presence of Hydrogen. Further understanding is developed by comparison with kinetic models to deconvolute the variables affecting Ostwald Ripening. In order to study Ostwald Ripening at the single atom scale in Hydrogen, Platinum nanoparticles are used as a model system. Particle decay is found to be initiated by a lack of local single atoms, and a subsequent increase in single atom density suggests anchoring of atoms onto sites on the Carbon substrate. These observations build an atomic level understanding of Ostwald Ripening, informing both traditional nanoparticle, and the emerging field of single atom, catalysis.