We use the rotating ring disk (RRDE) method to study activity–selectivity relationships for the oxygen reduction reaction (ORR) on Pt(111) modified by various surface coverages of adsorbed CN ad (Θ CNad ). The results demonstrate that small variations in Θ CNad have dramatic effect on the ORR activity and peroxide production, resulting in “volcano-like” dependence with an optimal surface coverage of Θ CNad = 0.3 ML. These relationships can be simply explained by balancing electronic and ensemble effects of co-adsorbed CN ad and adsorbed spectator species from the supporting electrolytes, without the need for intermediate adsorption energy arguments. Although this study has focused on the Pt(111)–CN ad /H 2 SO 4 interface, the results and insight gained here are invaluable for controlling another dimension in the properties of electrochemical interfaces. • Oxygen reduction reaction was studied in cyanide-modified Pt(111) electrodes. • The amount of adsorbed cyanide has a dramatic effect on the activity and selectivity. • A “volcano-like” dependence was obtained for ORR selectivity and activity. • Electronic and ensemble effects operate to control interfacial properties. • Interactions between multiple spectator species alter the activity and selectivity.