Laser-ablated cobalt atoms have been reacted with CO molecules during condensation in excess argon. The CoCO molecule is observed after deposition, and CO(CO)(2,3,4) are formed on annealing in agreement with earlier matrix work. Cobalt carbonyl anions are also produced and trapped. On the basis of isotopic substitution and density functional calculations, a sharp absorption at 1804.0 cm(-1) is assigned to the C-O stretching mode of CoCO-, 1768.9 and 1860.2 cm(-1) bands to antisymmetric and symmetric C-O stretching vibrations of bent Co(CO)(2)(-), and 1826.9 and 1890.0 cm(-1) absorptions to the antisymmetric stretching modes of Co(CO)(3)(-) and Co(CO)(4)(-), respectively.