In recent years, non-aqueous electrolytes received increasing interest for their application in the area of energy conversion and storage. While in aqueous electrolytes the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) usually limit the stability regime of the electrolyte, nonaqueous electrolytes allow studying processes beyond these potential limits. Among the non-aqueous electrolytes, ionic liquids (ILs) are promising candidates due to their high electrochemical and thermal stability. Furthermore, they are intrinsically ion-conducting and have low vapour pressures in contrast to other organic electrolytes considered for such kind of applications. To understand an electrocatalytic reaction, a detailed understanding of the surface redox processes with the supporting electrolyte is indispensable since the components from the electrolyte adsorbed on the surface or found in the electrolyte near the electrode can have a significant influence on the observed activity. Preferably, such studies are performed on welldefined single crystal electrodes, acting as a benchmark and allowing for direct compari-son between theory and experiment, as shown for example for Au(111), Cu(100), or Ag(111). A prominent model electrode is Au(111) since Au is rather inert and different preparation procedures yield reproducible clean surfaces. Cyclic voltammetry is well-suited to study the electrochemical interaction of ILs with different substrates. The features observed within their electrochemical stability window can be attributed to a variety of processes, such as adsorption processes or reductive decomposition of the anions. These electrochemical features can overlap with those induced by impurities, such as residual halide and alkali metal ions, originating from the IL synthesis, residual oxygen, or residual water, or a combination of both. Interestingly, it was observed that current-voltage transients of an IL can be nearly featureless within the electrochemical stability window if the experiment is performed under vacuum conditions or in a purified Ar atmosphere. Studying the influence of residual water in ILs is an important aspect since water has a tremendous impact on electrocatalytic reactions, such as the oxygen reduction reaction (ORR), or metal deposition. Additionally, residual water can lead to unwanted side reactions or a decrease of the electrochemical stability window of the IL. The uptake of water depends on the particular IL and occurs in contact with water or even from the surrounding atmosphere. It has been shown that this is the case for both hydrophilic and hydrophobic ILs. Thus, in order to improve the reproducibility of the obtained system behaviour when using ILs as electrolytes, purification steps that reduce the amount of water, oxygen and other volatile compounds seem mandatory. This can, for example, be achieved by a combination of heat and vacuum treatments. Another purification method involves binding of the impurities to a solid phase such as activated carbon or a molecular sieve. Molecular sieves are made of zeolites and their pore dimensions determine their selectivity in removing certain types of differently-sized impuri-ties. For instance, detailed studies revealed that molecular sieves of 3 --4 A pore size are optimal for removing water. However, even if molecular sieves are thought to be rather clean and able to remove water from the ILs, it should be noted that they can also introduce new unwanted impurities by ion exchange into the IL. Overall, the electrochemical behaviour of a certain IL on a specific electrode material is often hard to reproduce since the synthesis and purification of these chemicals is rather difficult. For example, it has been shown that even different IL batches from the same supplier can behave differently, as illustrated by a set of cyclic voltammograms (CVs) extracted from the literature in Figure . All CVs show the electrochemical behaviour of N -butyl-N -methylpyrrolidinium bis(trifluoromethane)sulfonimide ([BMPyr][TFSI]) on a Au(111) electrode. For comparison, the different curves were adjusted to a specific peak, which is explained in more detail in the data evaluation section of the SI. Figure ) shows two curves from the same publication, but with different potential limits. A similar behaviour is observed by comparing CVs recorded in [MPPip][TFSI] on Au(111) published by our group within the last five years (see SI Figure ). To understand these differences in more detail, in this work, we revisited the electrochemical properties of the very same electrolyte used previously ([MPPip][TFSI]) on Au(111). Due to its broad electrochemical stability window, this electrolyte was also used in former studies related to metal deposition. Using cyclic voltammetry, we systematically studied the electrochemical behaviour of different batches of the same IL, ordered on different dates from different suppliers. In particular, we explored how the CVs changed by varying the lower potential limit and after recording multiple potential cycles with the same potential limits. Similar experiments were performed with ILs pre-treated in different ways, i.e., drying the IL in a vacuum and at elevated temperatures, adding molecular sieves after this procedure, or by increasing the water content, which inevitably also introduces trace amounts of oxygen. The role of other possible contaminants originating from the manufacturing process or from the addition of the molecular sieve is discussed as well.