Abstract The microwave spectra of CH3CH2SiH2CH3 and CH3CH2SiD2CH3 in the trans form were studied. The rotational constants calculated from the spectra can be explained satisfactorily with a model which has structural parameters transferred with only a slight modification, from the values reported for dimethylsilane and ethylsilane. From the observed splittings of the spectra, the barrier to the internal rotation of the SiCH3 group was calculated to be 1520 ± 30 cal/mol, assuming that the coupling effect of the torsional motions of the other groups can be ignored. From the Stark shifts, the dipole moment was determined to be μa=0.036±0.030, μb=0.757±0.003, and μtotal=0.758±0.005 D, making an angle of 2°44′ with the b axis for the parent species, while it was determined to be μa=0.024±0.030, μb=0.758±0.003, and μtotal=0.758±0.005 D, making an angle of 1°49′ with the b axis for the deuterated species. From these results, the orientation of the dipole moment in the molecule was concluded to be along the direction slightly inclined from the bisector of the CSiC angle toward the ethyl group in the molecule.