The rotational spectra of the methanol (\cdot) Ar complex and five of its isotopomers were reinvestigated by Fourier transform microwave spectroscopy. New (E) internal rotation state transitions were measured and fit to determine the potential barrier that hinders the methyl group internal rotation for all the species. A dramatic reduction of this torsional barrier height determined using the conventional internal rotation Hamiltonian (from (373 \mathrm{~cm}^{-1}) in bare methanol to about 68.5 (\mathrm{cm}^{-1}) in (\mathrm{CH}_{3} \mathrm{OH} \cdot \mathrm{Ar}) ) was observed. It was proposed that this apparent reduction arises from neglect of the large amplitude hydroxyl hydrogen wagging motion (or its hindered internal rotation) relative to argon. Assuming that the methyl group torsional barrier does not change upon complexation, an estimate for the hydroxyl torsional barrier of about (130 \mathrm{~cm}^{-1}) was obtained. The rotational constants obtained for various isotopomers of the complex were also used to obtain an improved structure of the complex. The structure and potential function calculations suggest that the hydroxyl hydrogen is oriented toward the argon as if weakly hydrogen bonding to the electron-rich rare gas atom. 1995 Academic Press. Inc.