Kinetic parameters and conformational equilibrium constants characterizing the internal rotation in gaseous N-ethyl, N-methylthioformamide (EMSF), N-ethyl, N-methylthioacetamide (EMSA), and N-ethyl, N-methyltrifluorothioacetamidc (EMSTFA) are determined:from exchange broadened H-1 NMR spectra. The Delta G double dagger(298) values, in kcal mol(-1), are 22.6 (0.1), 17.8 (0.1), and 17.2(0.1) for EMSF, EMSA, and EMSTFA, respectively. Conformer Gibbs free energy differences, Delta G degrees(Delta G degrees = G degrees (methyl group syn to thionyl sulfur) - G degrees (methyl group anti to thionyl sulfur)), in cal mol(-1), are -123 (24), 56 (9), and 96 (13) for EMSF, EMSA, and EMSTFA, respectively. These results are compared to solution phase values and to the corresponding oxoamides.