Understanding the impact of composition and interfaces between metals and oxides is a goal of interest for many chemical reactions. Herein, we propose a framework to map correlations between the electrochemical behavior of the oxide and the stability and reactivity of metal|oxide interfaces, exemplified by Cu|oxide for the electrochemical CO2 reduction reaction (CO2RR). Copper materials interfaced with metal oxides have emerged as promising CO2RR catalysts for selectivity toward multicarbon products, including alcohols; stability under operation has been reported for some of them. However, design rules are currently lacking. Herein, we propose the synthesis of well-defined Cu-MOx core–shell nanoparticles to investigate and compare the behavior of Cu-ZrOx, Cu-MgOx, and Cu-TiOx. By tracking the speciation and morphological evolution of these model catalyst materials, we find that the cathodic stability of the formed interfaces is determined by the operating potential and phase stability of the pure oxides and by their chemical interaction with copper. We learn that the interplay between these factors shapes the restructuring pathways for Cu-MOx catalysts and eventually drives their selectivity in the CO2RR. The developed understanding can be applied beyond this reaction, and the developed nanomaterials can be used beyond catalysis.