Herein, we report the infrared multiple photon dissociation (IRMPD) depletion spectroscopy results for an isolated protonated dimer of cytosine. Interestingly, in contrast to the previous IRMPD action spectroscopy results,(14) strong and narrow IR absorptions at 2903 and 3455 cm(-1) clearly appeared, where N-H stretching vibrations in the intermolecular hydrogen bonds, -HN-H center dot center dot center dot O=C-, were predicted. The results show that the protonated dimer of cytosine in the gas phase is indeed triply bound and possesses three intermolecular hydrogen bonds, of which one is ionic. In addition, the appearance of intermolecular hydrogen bonding may offer further opportunities to study the base pairing interactions involved in the protonated dimers of nucleic acid bases.