The addition of carbon monoxide to the fluorosilicon cations SiF+, SiF2.+, and SiF3+ has been investigated in helium buffer gas at (294 +/- 3) K and (0.35 +/- 0.01) Torr using a selected-ion flow tube (SIFT) apparatus. The monofluorosilicon cation was found to be unreactive toward CO, whereas both the difluoro- and trifluorosilicon cations consecutively added two CO molecules. Molecular orbital calculations, using density functional theory (DFT) performed at the B-LYP/6-31G(d,p) level, showed that the lowest-energy isomer on the SiF3(CO)(2)(+) potential energy surface has a trigonal bipyramidal structure in which a pentacoordinate Si atom is surrounded by two axial CO ligands (bonded through C) and three equatorial F substituents. he ion at the global minimum on the SiF2(CO)(2)(.+) potential energy surface has a structure between that of a distorted tetrahedron and a trigonal bipyramid in which both CO molecules are axial and the two fluorine atoms and the unpaired electron are equatorial. Other low-lying isomers have trigonal bipyramidal structures in which one or both CO ligands are bonded to silicon through oxygenn or have tetrahedral structures in which an F3SiCO+ or F2SiOC.+ ion is solvated by CO. A multicollision-induced dissociation (CID) study of the product SiF3(CO)(2)(+) indicated the presence of at least three structural isomers, whereas the CID of SiF2(CO)(2)(.+) was less conclusive.