The rotational spectra of five isotopologues of dimethyl sulfate have been assigned by pulsed-jet Fourier-transform microwave spectroscopy. The global minimum configuration has a C(2) symmetry, with the two methyl groups nearly trans with respect to the two sulfonic oxygen atoms. The internal rotations of the two methyl groups split each transition into several components line, allowing for the determination of accurate values of the V(3) barrier height to internal rotation and of the orientation of the methyl groups with respect to the principal axis system. The geometry of the molecular skeleton is fully determined. (C) 2011 Elsevier B.V. All rights reserved.