CO2 electrochemical conversion to useful energetic molecules such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH) and methane (CH4) is of vital importance. Ionic liquids (ILs), which are defined as salts consisting of organic cations and/or inorganic anions which melt at or below 100 °C, possess many physicochemical advantages such as high CO2 solubility, suitable ionic conductivity, wide electrochemical potential window, high thermal stability and negligible vapor pressure.1 Imidazolium based room-temperature ILs (RTILs) and their mixtures with molecular solvents have been used for CO2 reduction, since they suppress the side reaction (H2 evolution) and may act as a cocatalyst. In this case, the Scanning Electrochemical Microscope (SECM)2,3 tip has been replaced by an optical fiber connected to an Xenon lamp to locally illuminate semiconductor materials, either with UV-Visible or Visible light.4 We use this approach to study CO2 photo-reduction reaction by illuminating p-type semiconductor oxides such as CuCo2O4, which are immersed in different binary RTILs/water mixtures including [C4mim][BF4]/H2O and [C2mim][BF4]/H2O. Our SECM results allow to compare CO2 photoelectrochemical reduction performance in different media. On the one hand, we get a relevant increase in photocurrent in 25% [C2mim][BF4]/75% H2O mixture in comparison with all other media tested either under UV-Visible or Visible illumination, pointing out the cocatalyst role of this imidazolium cation for the photoelectrochemical reduction of CO2. On the other hand, in contrast, we find a decrease in photocurrent when 25% [C4mim][BF4]/75% H2O mixture is used, probably due to the strong interaction of [C4mim]+ and CO2,which avoids further reduction of CO2. References [1] Sánchez-Sánchez, C. M. Electrocatalytic Reduction of CO2 in Imidazolium-Based Ionic Liquids. In: Wandelt, K. (Ed.), Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry. 2018, vol. 5, pp 539–551. [2] Bard, A. J.; Mirkin, M. V. (Eds.), Scanning Electrochemical Microscopy; CRC Press, 2012. [3] Sun, P.; Laforge, F. O.; Mirkin, M. V. Scanning Electrochemical Microscopy in the 21st Century. Phys. Chem. Chem. Phys. 2007, 9 (7), 802–823. [4] Lee, J.; Ye, H.; Pan, S.; Bard, A. J. Screening of Photocatalysts by Scanning Electrochemical Microscopy. Anal. Chem. 2008, 80 (19), 7445–7450.