-at the atomic level -a property that is well documented . Additionally, the HEA Seg sample exhibited a higher OR compared to the HEA Mix, suggesting a positive effect of the atomic-level degree of mixing on oxidation resistance. To understand this trend, it is essential to consider the oxophilicity of the metals involved. The term oxophilicity refers to the tendency of an element to form oxides. Specifically, the oxophilicity values for Ru, Ir, and Rh are 0.4, 0.4, and 0.3, respectively, which are significantly higher than those for Pt and Pd, which are 0.1 and 0.0, respectively . This greater propensity to form oxides may be contributing to the elevated levels of OR observed for the Ru-Ir, Ru, and Ru-Rh samples compared to Ru-Pd and Ru-Pt ones. Thus, the decrease in OR when increasing compositional might simply be related to the fact that the Ru content decreases when adding other metals (from 100% to 20% as sample complexity increases). Since Ru is the most oxophilic metal in the series, it is no surprise that the overall OR decreases with the diminution of Ru. To mitigate the impact of the differences in Ru loadings across the samples set, the individual OR of each different metal (except Ru) were examined. The Oxidation Ratio of one metal M is defined as: This analysis revealed the same decreasing trend (Figure ). The binary alloys consistently showed the highest OR values, and this value decreased when compositional complexity increases, with HEA Mix showing the lowest OR values. One exception to this trend was found for Pt (Ru-Pt-Pd-Ir alloy has a higher OR value than the Ru-Pt-Pd alloy, and HEA Mix shows a higher OR value than HEA Seg). This could indicate an abnormal effect of the atomic-level degree of mixing on Pt oxide formation. Statistical assessments were conducted using pairwise t-tests (see Section S6.2); the differences were statistically significant.