Indeed, the seemingly single FCC phase in the HEA could either result from a single phase containing all five elements or from the HEA being constituted of small pure FCC crystallites, each made of one single element. This ambiguity explains why the diffractograms of the HEA Seg and Mix are similar and constitutes one of the motivations to use other techniques capable of revealing the true atomic-level mixing. The samples were then analyzed by ToF-SIMS and the data were processed to extract useful insights on the local surface chemistry, in particular on the degree of elemental mixing and on the oxidation resistance.