The lowest frequency degenerate fundamental band of CH 3 SiD 3 (v 12 ϭ 1 4 0) centered around 418 cm Ϫ1 was measured in order to investigate the vibration-torsion-rotation interactions in a symmetric-top molecule with a single torsional degree of freedom. The spectrum was recorded at an instrumental resolution of 0.004 cm Ϫ1 using a Bomem Fourier transform spectrometer. The temperature and pressure of the sample were 180 K and 2 Torr, respectively. Because of the Coriolis coupling between the torsional stack with one quantum of the silyl rock excited and the corresponding stack for the ground vibrational state, torsional splittings are measured that are substantially larger than expected simply from the observed increase in the barrier height. Due to the local nature of the Coriolis perturbation, the significantly enhanced torsional splittings are confined to a few (K, ) rotational series; here ϭ Ϫ1, 0, 1 labels the torsional sublevels. The current measurements of the 12 band and frequencies from previously reported studies in the ground vibrational state were fitted to within experimental uncertainty using an effective Hamiltonian which was used for the analyses of similar spectra in CH 3 SiD 3 and CH 3 CD 3 . Spectroscopic parameters characterizing the states v 12 ϭ 0 and 1 and their interactions were determined, including several Coriolis-coupling constants.