Heterogeneous catalysts have been pivotal to the development of the modern chemical industry and are essential for catalysing many industrial reactions. However, reaction rates are different for every individual catalyst particle and depend upon each particle s morphology and size(1), crystal structure(2) and composition(3-7). Measuring the rates of reaction on single nanocrystals will enable the role of catalyst structure to be quantified. Here, using surface plasmon spectroscopy, we have directly observed the kinetics of atomic deposition onto a single gold nanocrystal and also monitored electron injection and extraction during a redox reaction involving the oxidation of ascorbic acid on a gold nanocrystal surface. These results constitute the first direct measurement of the rates of redox catalysis on single nanocrystals.