In this paper, we show that time-of-flight secondary ion mass spectrometry (ToF-SIMS) can be used to demonstrate that alloying exists at the atomic scale in these alloys and to quantitatively assess the atomic level degree of mixing at the surface of HEAs. ToF-SIMS provides detailed chemical information about the top atomic layers of a sample . The technique involves bombarding the sample surface with a pulsed primary ion beam under ultrahigh vacuum conditions , generating secondary ions that carry chemical information about the surface and that can be separated based on their masses. ToF-SIMS was proven powerful for the characterization of heterogeneous catalysts, e.g. to probe the dispersion of active elements , to assess the interaction between different components , or to study the cause of deactivation . ToF-SIMS has already been used to analyze sample composition with a spatial resolution of 20 nm in width . Additionally, ToF-SIMS has been utilized to gather information about the correlated presence of two metals in mixed oxide nanoparticles with sizes down to a few nanometers . In all these studies, sputtered cluster ions are used to assess the intimate mix of their elemental constituents in the solid. The rationale is that the craters formed by energetic cluster ions, such as 30 keV Bi3-5 + , in inorganic solids are only a few nm in diameter. Since clusters must be formed within a single impact event, their constituents must imperatively arise from the same crater nanovolume. Moreover, molecular dynamics simulations indicate that clusters containing more than 2-3 atoms are often emitted as such (i.e. without recombination above the surface), implying that they were direct neighbors in the solid .