The boron carbonyl cluster cations in the form of B-3(CO)(n)(+) (n = 4-6) are produced and studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region in the gas phase. Their geometric structures are determined with the aid of density functional theory calculations. The B-3(CO)(4)(+) cation is characterized to have a D-2d (OC)(2)B=B=B(CO)(2) structure and (1)A(1) electronic ground state with a linear boron skeleton. The B-3(CO)(5)(+) cation is determined to have a chain boron framework with C-2 nu symmetry. The B-3(CO)(6)(+) cation is a weakly bound CO-tagged complex involving a B-3(CO)(5)(+) ion core. Bonding analysis reveals that B-3(CO)(4)(+) has a chemical bonding pattern similar to allene, while bonding in B-3(CO)(5)(+) is similar to that in allyl anion.