Research on entropy stabilized oxides (ESOs) has primarily focused on exploring new structures, chemistries, or unique properties. However, few studies discuss the impact of secondary phases on functionality. Here, electrical transport mechanisms in the canonical ESO (Co,Cu,Mg,Ni,Zn)O were assessed as a function of secondary phase content . When single-phase, the oxide is an oxygen ion conductor. After heat treatments, Cu-rich tenorite particles form at grain boundaries, turning the grain interior rocksalt oxide into a mixed ionic-electronic conductor via oxygen ions and small hole polarons. While the grain interiors exhibit similar mixed ionic-electronic conduction, Cu-rich tenorite grain boundary phases create a pathway for Cu2+/Cu3+ small hole polarons. The ability to selectively grow secondary phases nucleated at grain boundaries enables tuning of electrical properties in entropy-stabilized and complex concentrated oxides using microstructure design, nanoscale engineering, and heat treatment, paving the way to develop many novel materials.
 Vahidi…Bowman (In Review)