Nu(CO) frequencies and relative intensities, and HE(I) photoelectron ionization energies were measured for some methyl alpha-hetero-substituted acetates (XCH2C(O)OMe: X = F, OMe, NMe2, Cl, Br, SMe, and I). The IR data indicate a cis-gauche rotational isomerism, where the gauche rotamers' stabilities should be due to a hyperconjugative interaction. The carbonyl frequency shifts of the cis rotamers have been ascribed to the Repulsive Field Effect, while for the gauche rotamers a competition between, the inductive and hyperconjugative effects should occur. The anomalous behaviour of some substituted acetates (X = F, OMe, and SMe) was attributed to orbital interactions. A good linear correlation between the carbonyl oxygen lone pair (n'o) ionization energy and the substituent sigma(I) inductive parameter (excluding X = Br and OMe) was found, as a consequence of the predominance of the inductive effect over the through-bond and through-space orbital interactions.3723745