Since we observed that reducing the amount of water in the IL has no significant effect on the electrochemical properties, we tried to increase the amount. The addition led to an increase of the cathodic peaks C1 for type I and peaks C2 and C4 for type II (see Figure ) (though we speculated that peak C1 overlaps with peak C2 in type II). We found that independent of the amount of water added (100 ppm to 400 ppm), the first potential cycles showed a very similar behaviour. However, storing the IL in a glove box for several hours or even days, leads to a gradual decrease of peak C1 of type I (see Figure )) even though the water content remained unchanged. Therefore, peak C1 must be induced by another species that appeared in the IL along with water. A likely candidate is O 2 since the water added to the IL was not deaerated beforehand. We assume that in the glove box, O 2 will gradually be released from the IL. With an IL containing 400 ppm water, this process took between two and five days. A strong difference was already observed after 12 hours. This is an important result as it indicates that the reproducibility of the electrochemical measurements strongly depends on how fast the experiments are performed or how long the chemicals are stored in the glove box before use (to be in equilibrium with the glove box atmosphere). A more detailed possible role of O 2 on the CV will be discussed below.