The rotational spectrum of the 1:1 weakly bound complex between dimethyl ether (DME) and CS2 has been measured by Fourier-transform microwave spectroscopy. Ab initio calculations at the MP2/6-311++G(2d,2p) level predict the lowest energy structure to have an ac-plane of symmetry with the CS, lying approximately along the lone pair direction of the DME (R((OS)-S-...) distance = 3.05 angstrom). Minor adjustment of this structure closely reproduces the experimental rotational constants. The CS2 tunnels between the lone pairs of DME, at a frequency of 90.3411(14) MHz, through a barrier of 78(1) cm(-1). (c) 2005 Elsevier B.V. All rights reserved.