A series of saturated ruthenium cluster carbonyls (Ru(CO)₅⁺, Ru₂(CO)₉⁺, Ru₃(CO)₁₂⁺, Ru₄(CO)₁₄⁺, Ru₅(CO)₁₆⁺ and Ru₆(CO)₁₈⁺) have been synthesized in the gas phase and subsequently characterized by infrared spectroscopy. Their size-specific IR spectra in the region of the carbonyl stretch vibration (1900–2150 cm⁻¹) and in the region of the Ru–C–O bending modes (420–620 cm⁻¹) are obtained by infrared multiple photon dissociation spectroscopy. The structures of these cluster carbonyls are assigned by comparison with results from density functional calculations. A multitude of differently activated CO ligands are identified in these cationic cluster carbonyls, reaching from terminal, over non-symmetrically bridging (semi-bridging) ligands with varying degrees of interaction to additional Ru atoms towards symmetrically bridging CO ligands.