ABSTRACTWe report results on the dynamics of tunnelling switching based on a high-resolution spectroscopic investigation of meta-D-phenol in GHz and THz ranges. The pure rotational spectra were recorded in the range of 72–117 GHz and assigned to the localized syn- and anti-structures in the ground and the first excited torsional states. Specific torsional states were unambiguously assigned by comparison of the experimental rotational constants with theoretical results from quasiadiabatic channel reaction path Hamiltonian (RPH) calculations. The torsional fundamental νT at ≈ 309 cm−1 and the first hot band (2νT – νT) at ≈ 277 cm−1 were subsequently assigned in synchrotron based high-resolution Fourier transform infrared (FTIR, THz) spectra. The analyses provided accurate spectroscopic constants of all six states involved. It was found that the 2νT states are interacting through anharmonic resonances, indicating tunnelling switching as predicted by theory. Furthermore, tunnelling–rotation–vibration transitions were assigned and the tunnelling splitting in 2νT was determined as 1.72450(17) cm−1. This key result allowed the assignment of two Q branches at 275.21303(9) and 277.67127(9) cm−1 to vibration-tunnelling transitions of the (syn ← anti) type, hence confirming tunnelling switching dynamics in m-D-phenol. The ground-state energy difference of the syn- and anti-isotopomers is obtained experimentally as E0 (syn) - E0 (anti) = (hc)0.82 cm−1 in satisfactory agreement with the theoretical ab initio predictions of (hc)1.5 cm−1 given the small absolute values arising from the tiny zero point energy effects. The results are discussed in relation to further fundamental aspects of tunnelling in slightly asymmetric potentials including the effects of the parity violating electroweak interaction in chiral molecules.