The carbonyl stretching frequencies and the apparent molar absorptivities are reported for some ω-ethylthio-p-substituted acetophenones and for the corresponding unsubstituted acetophenones. In the ω-ethylthio-series the solvent effect indicates the presence of thecis/gaucherotational isomerism, being thegaucherotamer predominant in the whole series (ca. 90%). In the ω-ethylthioacetophenones, on going from electron-attracting to electron-donating substituents, the progressive increase in the carbonylcisshifts (Δνc) is attributed to the increasing Field Effect between C˭O and C‒S dipoles, and the progressive decrease in the carbonylgaucheshifts (Δνg) is due to the deacreasing contribution of the hyperconjugative interaction between πCOand σc‒sorbitals. The U.V. spectra of the ω-ethylthioacetophenones display an→ π*COband which is bathochromically shifted in relation to the corresponding acetophenones, indicating the occurrence of the hyperconjugative interaction between π*COand σ*c‒sorbitals in the excited state.13C NMR data for the methylene group of ω-ethylthioacetophenones, compared to those for some ω-haloacetophenones,2,3suggest that in the fundamental state besides the hyperconjugative interaction, there is also the occurrence of charge transfer from πCOto 3d(S)orbitals.