The analysis of the nu(CO) bands in the IR spectra of the S-ethyl alpha-heterosubstituted thioacetates helped by Molecular Mechanics Calculations indicated the existence of the cis/gauche rotational isomerism. In the thioacetates series the gauche rotamers predominate over the cis ones. This has been ascribed to the large stabilization of the pi(CO)* orbital leading to a stronger pi(CO)*/sigma(C-X) and pi(CO)*/n(X) orbital interactions and to a larger stabilization of the gauche rotamers. The positive carbonyl cis shifts (DELTAnu(c)) ascribed to the Repulsive Field Effect are smaller than the ones for the heterosubstituted-acetones, -acetophenones, -methyl acetates and -N,N-diethylacetamides. This trend is discussed in terms of the lower carbonyl dipole moment for the thioesters which is mainly due to the pi(CO)/3d(s) interaction in the carboxythioethyl group. The larger negative carbonyl gauche shifts (DELTAnu(g)) for the heterosubstituted thioacetates in relation to the other referred carbonyl compounds along with the higher Non Additivity Effect of the alpha-methylene carbon chemical shifts for the majority of the title compounds in comparison with the other hetero-substituted carbonyl compounds are further support for the larger contribution of the pi(CO)*/sigma(C-X) and pi(CO)*/n(X) orbital interactions in the thioesters than the same interactions for the other carbonyl compounds. This has been attributed to a great stabilization of the pi(CO)* orbital in the carboxythioethyl group of the thioester due to its interaction with a 3d sulfur orbital.7141730107125