IR ν_CO and ν_SO frequencies of some α-sulfinylacetophenones [PhC(O)CH₂S(O)R: R = Me 1, Et 2, Prⁱ 3, Ph 4 and Bu^t 5] have been measured and their conformations are estimated with the help of ab initio 6-31G** calculations and X-ray diffraction analyses. The anomalous negative carbonyl frequency shifts for the cis₂ rotamer together with the decrease of the cis∶gauche population ratio in solvents of increasing polarity for compounds 1–4 support the existence of a strong intramolecular interaction between CO and SO dipoles, which stabilizes the cis₂ rotamer more than the π_CO–σ*_C–SO and π*_CO–σ_C–SO orbital interactions stabilize the gauche₃ rotamer. The stability of the cis₂ rotamer is discussed in terms of the electrostatic attraction between the CO and SO dipoles along with the π*_SO←n_O(CO) charge transfer which lead to an O_(CO)· · ·S_(SO) contact shorter than the sum of the corresponding van der Waals radii. The gauche₂ rotamer of 5 is more stable than the cis₂ one in which steric strain between the carbonyl oxygen atom and the tert-butyl group is present.