Spectroscopic evidence for clusters of like charged ions in ionic liquids In this work the counterintuitive phenomenon of attraction between ions of like charge is described for ionic liquids. It is based on cooperative hydrogen bonding leading to different sized cationic clusters. Experimentally these clusters are indicated by characteristic spectroscopic features like e.g. redshifts of OH vibrational frequencies. The experimental observables can be related to charge transfer and second order stabilization energy. Finally a theoretical approach of stabilizing the cationic cluster network by either neutral molecules or negatively charged counterions has been done.<eng>