Conversion of CO2 into chemicals and materials with an higher energy content makes CO2 electrolysis interesting for sustainable energy storage. One major strategy is to use electrochemical CO2 reduction reactions (CO2RR) in order to convert CO2 into CO, alcohols and formic acid. However, high overpotentials, competitive hydrogen evolution reaction (HER), as well as low product selectivity and Faraday efficiency, impairs CO2 electrolysis for applications so far. In order to reduce existing overpotentials, the use of room-temperature ionic liquids (RTILs) as promising electrolytes has gained much attention. That is because of their wide electrochemical window, excellent electric conductivity and high CO2 solubility. However, the reaction mechanism that is responsible for the drastically lower onset potentials for CO production from CO2RR, is for imidazolium based RTIL still controversial. Using in operando IR absorption and in situ sum-frequency generation (SFG) spectroscopy, we provide new information on the role of the imidazolium cations for CO2RR. For this purpose, we have studied Pt electrodes in contact with 1-butyl-3-methylimidazolium [BMIM], 1-butyl-2,3-dimethylimidazolium [BMMIM] and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMPyrr][NTf2] RTILs that contained 10 to 500 mM H2O. Although, Pt catalysts have clear disadvantages in terms of costs and catalyst performance for CO2RR, because CO forms at low overpotentials but leads to catalyst poisoning and deactivation, the resulting limited number of side reactions renders Pt catalysts interesting for spectroscopic investigations that focus on the molecular origin of the existing low overpotentials. Cyclic voltammetry shows that Pt is active for CO2RR in all three RTILs, with the highest current densities in case of [BMMIM][NTf2]. SFG spectroscopy, demonstrates that CO forms at an onset potential of -0.7 V vs SHE, when [BMMIM] or [BMPyrr] cations and [NTf2] anions are present. This onset potential is independent of the H2O concentration and we propose that CO formation can occur predominantly via an electrostatically stabilized CO2 radical anion. On the other hand, in [BMIM][NTf2] electrolytes a strong dependence on the H2O concentration is observed for potentials <-0.4 V where we show that cations with an active C2 position at the imidazolium ring can act as co-catalysts that enables CO formation through a reactive imidazolium-2-carboxylic acid intermediate.