Infrared spectra of mass-selected homoleptic cobalt carbonyl cluster cations including dinuclear Co-2(CO)(8)(+) and Co-2(CO)(9)(+), trinuclear Co-3(CO)(10)(+) and Co-3(CO)(11)(+), as well as tetranuclear Co-4(CO)(12)(+) are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The geometric structures of these complexes are determined by comparison of the experimental spectra with those calculated by density functional theory. The Co-2(CO)(8)(+) cation is characterized to have a Co-Co bonded structure with C-5 symmetry involving a bridging CO ligand. The Co-2(CO)(9)(+) cation is determined to be a mixture of the CO-tagged Co-2(CO)(8)(+)-CO complex and the Co(CO)(5)(+)- Co(CO)(4) ion molecular complex. The Co-3(CO)(10)(+) cation is the coordination-saturated trinuclear cluster, which is characterized to have a triangle Co-3 core with C-2 symmetry involving two edge-bridging and eight terminal CO ligands. The Co-3(CO)(11)(+) cation is a weakly bound complex involving a Co-3(CO)(10)(+) core ion. The Co-4(CO)(12)(+) cluster cation is deduced to have a tetrahedral Co-4(+) core structure with three edge-bridging and nine terminal carbonyls.