Carbonyl core (C 1s, O 1 s)-->pi*(C=O) transitions are distinctive in the near-edge X-ray absorption fine structure (NEXAFS) spectra of species containing carbonyl groups. These features are used for the chemical microanalysis of organic materials using X-ray microscopy. We have explored the chemical sensitivity of these features in C 1s and O 1 s NEXAFS spectra for a series of polymers containing the carbonyl group in a range of different bonding environments. Ab initio calculations are used to explain the origin of the observed trends and to explore the effect that orbital interactions have on the energy of these core (C 1s, O 1s)-->pi(C=O) features. The differences between the experimental and the calculated carbonyl core (C 1s, O 1s)-->pi(C=O) transition energies are systematic and can be used to develop a semiempirical method for predicting the absolute (experimental) transition energies from the calculated transition energies. This relationship is applied to a large body of calculated transition energy data to prepare correlation diagrams for the carbonyl (C 1s, O 1s)-->pi(C=O) transitions. These correlation diagrams will be useful for the analytical application NEXAFS spectroscopy to organic materials.