The rotational spectrum of (\left(\mathrm{CH}{3} \mathrm{OH}\right){2}) has been observed in the region (4-22 \mathrm{GHz}) with pulsedbeam Fabry-Perot cavity Fourier-transform microwave spectrometers at NIST and at the University of Kiel. Each (a)-type (R(J), K_{a}=0) transition is split into 15 states by tunneling motions for (\left(\mathrm{CH}{3} \mathrm{OH}\right){2},\left({ }^{13} \mathrm{CH}{3} \mathrm{OH}\right){2},\left(\mathrm{CH}{3} \mathrm{OD}\right){2},\left(\mathrm{CD}{3} \mathrm{OH}\right){2}), and (\left(\mathrm{CD}{3} \mathrm{OH}\right){2}). The preliminary analysis of the methyl internal rotation presented here was guided by the previously developed multidimensional tunneling theory which predicts 16 tunneling components for each (R(J)) transition from 25 distinct tunneling motions. Several isotopically mixed dimers of methanol have also been measured, namely ({ }^{13} \mathrm{CH}{3} \mathrm{OH}, \mathrm{CH}{3} \mathrm{OD}, \mathrm{CD}{3} \mathrm{OH}), and (\mathrm{CD}{3} \mathrm{OD}) bound to ({ }^{12} \mathrm{CH}{3} \mathrm{OH}). Since the hydrogen bond interchange motion (which converts a donor into an acceptor) would produce a new and less favorable conformation from an energy viewpoint, it does not occur and only 10 tunneling components are observed for these mixed dimers. The structure of the complex is similar to that of water dimer with a hydrogen bond distance of (2.035 \AA) and a tilt of the acceptor methanol of (84^{\circ}) from the (\mathrm{O}-\mathrm{H}-\mathrm{O}) axis. The effective barrier to internal rotation for the donor methyl group of (\left(\mathrm{CH}{3} \mathrm{OH}\right){2}) is (V{3}^{\prime}=183.0 \mathrm{~cm}^{-1}) and is one-half of the value for the methanol monomer ( (370 \mathrm{~cm}^{-1}) ), while the barrier to internal rotation of the acceptor methyl group is 120 (\mathrm{cm}^{-1}). C1995 Academic Press, Inc.