The results of free jet millimeter-wave absorption and molecular beam Fourier transform microwave investigations of dimethyl ether-Ar are reported. The Ar atom lies in the sigma(v) symmetry plane of dimethyl ether at a r(0)-distance of 3.58 A and the line connecting the argon atom to the center of mass (cm) of dimethyl ether (Ar-cm) forms a r(0)-angle of 77degrees with the O-cm line. All rotational transitions are split into two component lines, due to the tunnelling of Ar from above to below the C-O-C plane. The corresponding inversion splitting has been used to determine the barrier to inversion. Some additional information on the pathways of the Ar inversion and the potential energy function have been obtained from ab initio calculations. (C) 2002 Elsevier Science B.V. All rights reserved.