Reactions of laser-ablated late lanthanide atoms (Th, Dy, Ho, Er, Tm, Yb, and Lu) with dilute carbon monoxide molecules in solid argon have been investigated using matrix-isolation infrared spectroscopy. The Ln(2)[eta(2)(mu(2)-C, O)](x) (Ln = Tb, Dy, Ho, Er, Lu; x = 1, 2) molecules are observed upon sample annealing, whereas no product is observed for Tm and Yb. The C-O stretching frequencies in these dilanthanide carbonyls range from 1100 to 1300 cm(-1), far below the value of free CO in the gas phase (2143.5 cm(-1)), implying that the C-O bonds are highly activated. Density functional theory calculations have been performed on these products. These Ln(2)[eta(2)(mu(2)-C, O)], molecules are predicted to have planar structures, which carry asymmetrically bridging CO moieties that are tilted to the side.