Thereby, we can demonstrate that increasing composition complexity (and therefore the entropy) leads to (i) more homogeneous atomic mixing at the atomic scale and (ii) higher resistance against surface oxidation (i.e. passivation). To test this hypothesis, two models porous HEAs based on Ru, Pt, Pd, Ir, and Rh are used as the case study. One well mixed at the atomic scale level (denoted "HEA Mix") and another showing phase segregation (denoted "HEA Seg"). A series of less complex alloys based on the same metals were also analyzed as benchmarks: pure Ru, binary Ru-Pt, Ru-Pd, Ru-Ir and Ru-Rh alloys, ternary Ru-Pt-Pd alloy, and quaternary Ru-Pt-Pd-Ir alloy (Table ).