The ILs can be pre-treated with a molecular sieve to further reduce the amount of water and simultaneously remove other impurities. In this work, a molecular sieve with pore sizes of 4 Å was added to the IL before drying in vacuum at 80 °C. The CVs for Au(111) recorded in ILs after this pretreatment compared to those recorded in the as-received ILs are shown in Figure b with solid and dashed lines, respectively. The maximum current density of peak C1, related to O 2 , decreases (see below). The reason for the decrease of peak A1 could so far not be determined.[ref-chemRxiv] More importantly, three additional faint features marked with stars appear at potentials more negative than -1.0 V in the negative-and positive-going scans. Two possibilities could rationalise the emergence of these features. First, the molecular sieve indeed removed some impurities which could block sites for other surface redox processes, or second, new impurities were introduced into the IL during the pre-treatment with the molecular sieve. A possible ion exchange of Na from the molecular sieve with cations from an IL was suggested previously by Ntais et al., based on X-ray diffraction and Raman spectroscopy measurements. To verify the hypothesis that new impurities were introduced (Na and/or others) we performed XPS measurements. The XP survey spectrum of an IL pre-treated by a molecular sieve is shown in the SI in Figure by the blue line. The XP spectrum contains features related to Si, where the Si 2p region is highlighted in Figure in blue. The grey line is the reference spectrum recorded on the as-received IL. In some cases, faint features of Na are observed as shown in the Na 2s region in Figure in blue. Both Si and Na are components of the molecular sieve. To unambiguously verify the presence of Na, we polarised a polycrystalline Au electrode at -1.8 V for 2 h in an IL pretreated with a molecular sieve to deposit possible Na contaminations from the IL,, i.e., to increase the local concentration of Na compared to that in the IL. Subsequently, the electrode was removed and the Au surface was rinsed with acetone to remove the IL. The survey XP spectrum of the Au surface after this treatment is shown in Figure in red. No signs for Si were observed on the electrode, but the Na 2s peak significantly increased, as shown in Figure 3b in red. Hence, the pre-treatment of an IL with a molecular sieve might not be the most suitable solution for further purification.