In carbonyl compounds RCOX (X = H, Me, SiR3,SR, Cl, F, OMe, OH, NH2, O-; R = H, Me), the O-17 shift values of the carbonyl group depend on the electron donor-acceptor properties of X, whereas the C-13 shift values are determined by other factors too. By IGLO ab initio calculations, the difference between C-13 and O-17 has been traced to differences in the relative importance of the shielding tensor component in the direction of the C-O bond (sigma(zz)). The deshielding contribution of this component is mainly determined by the energy of the n-pi* excitation; donor-acceptor-type interactions influence the level of both orbitals. As the n orbital of this transition is essentially localized on the O atom, the excitation acts less on C-13 and is thus mainly responsible for the difference in substituent sensitivity between C-13 and O-17 shifts. An analogous difference of substituent sensitivities between C-13 and O-17 shifts exists in aroyl compounds p-YC6H4COX towards changes in the para substituent Y; it is explained on the same basis as the effect of X upon RCOX, without the necessity of referring to a 'reverse' substituent effect. Finally, the surprising absence of substituent effects upon the O-17 shift in aroyl cations p-YC6H4CO+ can be explained by the fact that, for symmetry reasons, the n-pi*-type excitation is absent in linear compounds. (C) 1997 by John Wiley & Sons, Ltd