It is shown that Ar combines with CoCO (X (2)Delta) with the binding energy of 7.18 kcal/mol (CASPT2), 6.25 kcal/mol (RCCSD(T)), and 5.05 kcal/mol (UMPWPW91) in which a counterpoise correction and zero-point vibrational energy correction have been included. The Co-C-O bending frequency has been calculated, by the UMPWPW91 method, as 373.0 and 422.9 cm(-1) for CoCO and Ar-CoCO, respectively, the latter of which is in good agreement with the corresponding experimental frequency, 424.9 cm(-1), determined for CoCO isolated in solid argon.