A theoretical study has been carried out on C-13 and O-17 NMR shielding tensors in M(CO)(6) (M = Cr, Mo, and W), Fe(CO)(5), and Ni(CO)(4). The study was based on modern density functional theory (DFT) and gauge-including atomic orbitals (GIAO). The calculated shielding constants and tensors are in good agreement with the available experimental data. The difference in the isotropic shielding Delta delta = sigma(co) - sigma(M(CO)n) between free CO, sigma(CO), and CO as a ligand, sigma(M(CO)n) was analyzed in detail. It was shown that the stretch of CO on coordination as well as the back-donation from d(x) on the metal to pi*(CO) of the CO ligand has a positive contribution to the coordination shift Delta delta whereas the donation from the sigma(CO) HOMO of CO to the metal orbitals has a negative contribution to Delta delta.