During an electrocatalytic reaction bonds are broken and formed, and this requires that the reactants, the intermediates formed at the elementary reaction steps, and the products interact with a given number of surface atoms of the catalyst. Modifying the number of groups with an adequate number of surface atoms in a suitable geometric arrangement for a determined reaction step to proceed may affect the activity and/or selectivity of the catalyst. Although separating purely geometric atomic ensemble effects from electronic effects is not straightforward, the insights extracted from a detailed investigation of atomic ensemble effects can have a profound impact in the determination of electrocatalytic reaction mechanisms and in the design of more active and more selective electrocatalysts. This Minireview illustrates, using cyanide-modified Pt(111) electrodes as an archetype, how eliminating only one kind of site from the surface (the site-knockout strategy) by means of a regular array of inert adsorbates can be used to successfully study atomic ensemble effects in electrocatalysis. The possible consequences for the design of more efficient and more selective electrocatalysts are also commented on.