Literature data forδ(13C),δ(17O) andλn→π*maxfor carbonyl groups in various compounds are reviewed. In a wide variety of carbonyl compounds there is a linear relation betweenδ(17O) andgln→π*max. However, deviations from this general relationship occur for a group of ketones. The explanation is found in differences in electron-donating properties of the substituents at the carbonyl group, which cause changes in the electron density and in the polarity of the carbonyl group.