Significance Catalysts are essential for a sustainable future because they reduce the energy required in chemical processes and the emission of harmful and polluting compounds. Revealing the function and structure of a working catalyst is a challenging task but critical in order to prepare more efficient and higher-performing materials. Very often, active sites are formed by many atoms that cooperate to perform the catalytic function. It is therefore challenging to identify the structure of the active site. Here, we combine uniform nanocrystal catalysts and theory insights to reveal the active-site ensemble necessary for alkene combustion. This approach can be extended to reveal atomistic details of working catalysts for a variety of applications.