AbstractA series ofpara‐ andmeta‐substituted acetophenones were prepared which were highly labeled with18O at the carbonyl function. The natural abundance13C NMR spectra of the compounds were recorded and the18O‐isotope‐induced shifts of the carbonyl carbon were measured in order to determine possible substituent group electronic effects on the18O‐isotope shift. The isotope shifts were found to be correlated with a number of properties which demonstrate the molecular basis of the isotope shift. Good correlations were obtained for the isotope shift as a function of σp+, the carbonyl stretching frequency, the carbonyl group bond order, then→π**transition of the carbonyl group and the chemical shift of theipsocarbon. In contrast, no correlation was observed between the magnitude of the18O‐induced isotope shift and the13C chemical shift of the carbonyl carbon. These properties are discussed in connection with the theoretical basis of th