High-frequency discharge through CS2/Ar yielded CS, which was co-condensed with laser-ablated boron atoms at 4 K. The BCS and B(CS)(2) thiocarbonyl molecules were formed via the reactions of ground-state boron atoms with CS in solid argon. On the basis of isotopic shifts and splittings, absorptions at 881.6 and 860.8 cm(-1) are assigned to the C-S stretching vibrations of the linear BCS and (BCS)-B-11 molecules. The absorptions at 1772.3/1727.2 and 1021.6/1005.7 cm-1 are assigned to the B-C and C-S stretching vibrations of the linear B-11(CS)(2) and B-11(CS)(2) molecules, respectively. The BCS molecule has a (4)Sigma(-) ground state, and the B(CS)(2) molecule has a (2)Pi(u) ground state with the bonding similar to the corresponding boron carbonyls.