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IR νCO and νSO frequencies of some α-sulfinylacetophenones [PhC(O)CH2S(O)R: R 4;= 4;Me 1, Et 2, Pri 3, Ph 4 and But 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 C&z.dbd6;O and S&z.dbd6;O dipoles, which stabilizes the cis2 rotamer more than the πCO–σ*C–SO and π*CO–σC–SO orbital interactions stabilize the gauche3 rotamer. The stability of the cis2 rotamer is discussed in terms of the electrostatic attraction between the C&z.dbd6;O and S&z.dbd6;O dipoles along with the π*S &z.dbd6; O←nO(CO) charge transfer which lead to an O(CO)  · 4;· 4;·  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.