The rotational spectra of nine isotopomers of the methanol . sulfur dioxide van der Waals complex were observed with a pulsed molecular beam Fourier transform microwave spectrometer, Each rotational transition is split into an A-state (m = 0) and an E-state (m = +/-1) transition due to methyl top internal rotation effects. The A and E transitions show an additional inversion splitting ranging from a MHz to a few tens of MHz in seven of the isotopomers. The inversion splitting is absent in the two (SOO)-O-16-O-18 isotopomers. The center frequencies of the inversion doublets were used in a simultaneous fit of both the A- and E-state transitions, producing rotational constants which allowed a complete determination of the structure of the complex, Analysis of the moments of inertia indicate that the complex has a stacked structure, The center of mass distance between the two monomers is 3.08(5) Angstrom. The effective torsional barrier height is V-3 = 128.6(1) cm(-1) based on the assumption that the methyl group rotates against a heavy frame. The dipole moment is mu(T) = 1.94(3) D. The inversion motion is discussed based on effects on the splitting associated with isotopic substitution and the transition dipole direction.