As mentioned earlier, HEAs often display superior oxidation resistance compared to conventional alloys. In an attempt to quantify this trait using ToF-SIMS data, a new metric, the Oxide Ratio (OR), was formulated. The negative polarity was used in this case, because the yield for oxidic species is much higher than in the positive mode. Thus, OR is here defined as the ratio of the sum of all oxide ions (MOx -) present in the sample to the sum of all primary ions (M -) present in the sample. For example, for the HEA alloys (Ru-Pt-Pd-Ir-Rh), the OR is expressed as: The OR was calculated for all the samples, considering all metals (Figure and Section S6.1 in the electronic supplementary information for a discussion on the statistical significance of the trends discussed here). From this analysis, it emerged that OR markedly decreased when the number of elements in the alloy increases (with the notable exception of Ru vs Ru-Ir and Ru-Rh). This hints towards a correlation between oxidation resistance and the number of components in the alloy. In particular, HEA Mix alloy showed the lowest OR value, confirming