In this short communication the selection rules for transitions involving asymmetry split energy levels belonging to various torsional-vibrational–rotational states are put forward for the first time in asymmetrically deuterated methanol species (e.g., CH2DOH,CHD2OH, etc.). Unlike the parent symmetrical methanol species these rules depend on the parity of the torsional vibration states, It has been discovered that each mixed torsional vibrational state could be treated as independent vibrational states connected by the b- or c-type dipole moments. Due to the complexity of torsion and rotation, the energies for the K-rotational ladder seem to vary randomly for various torsional-vibrational states. These often result in inverted and/or large asymmetry splitting. In these regards, the aspects in which caution is rendered in the information available in the literature are briefly discussed. The findings are supported by rigorous assignment studies in the very high-resolution synchrotron radiation-assisted Fourier transform spectrum.