Is it possible to really 'see' individual molecules in action as they are involved in a chemical reaction at a surface? And can we, in this way, get a complete understanding of reaction mechanisms, at the resolution of atoms? The importance of studying chemical reactions at surfaces has recently been highlighted by Gerhard Ertl being awarded the Nobel Prize in chemistry in 2007, for elucidating mechanisms of chemical processes on heterogeneous catalysts at the single molecule level with Scanning Tunneling Microscopy (STM). Although ground-breaking, these studies were carried out in ultra-high vacuum, which is, however, an unrealistic condition for conventional chemical or biological reactions which usually occur in a liquid medium. The aim of this ERC proposal is to establish a research area at the interface of chemistry and physics which has so far been nearly completely unexplored: the investigation of chemical reactions at solid-liquid interfaces at the highest detail possible, by visualizing molecules with STM while they are involved in a reaction. By doing so, unique information about reaction mechanisms can be obtained by looking at individual molecules, instead of ensembles where the behaviour of many molecules is averaged.Towards this goal I propose to use a newly developed catalysis-STM setup, which is equipped with a liquid-cell and a bell-jar, and in which the conditions that are commonly applied in chemical laboratory processes (e.g. addition and withdrawal of chemicals, working under different atmospheres) can be closely resembled. In this setup I intend to carry out chemical reactions at a surface and monitor the behaviour of individual adsorbed catalysts, while they are in action. More specifically, it is my aim to investigate in detail the relation between structure and reactivity at the nanoscale