In oxide films on CoCrCuFeNi medium-entropy alloys with Cu contents of 0–‍5 at%, Cu existed as CuO, and the concentration increased with increasing Cu content in the alloys. Active dissolution in HCl was suppressed by the addition of Cu. The pitting potential in NaCl increased with the addition of 1 at% Cu, but it decreased with the addition of 3 at% Cu or more. The role of Cu was clarified by relating the active dissolution rate and pitting potential. In addition, the effects of Mo and W are discussed. •Copper was present as CuO in the oxide film, with a concentration twice the alloy content.•Dissolution of CuO from the films was responsible for suppressing active dissolution and decreasing pitting potential.•The addition of Mo or W improved the pitting corrosion resistance of the CoCrCuFeNi medium-entropy alloys.