The rotational spectra of three isotopomers of the Ar–dimethyl sulfide (DMS) complex – normal, 34 S, and 13 C species – were measured in the frequency region from 3.7 up to 24.1GHz by Fourier transform microwave spectroscopy. The normal species yielded 43 a-type and 79 c-type transitions. No Ar tunneling splitting was observed, while many transitions were split by the internal rotation of the two methyl tops of the DMS unit. In cases where the K-type splitting was close to that due to methyl internal-rotation, several forbidden transitions were observed that followed b-type selection rules. All of the observed transition frequencies were analyzed simultaneously using a phenomenological Hamiltonian also used in previously published work describing the Ar–dimethyl ether (DME) and Ne–DME complexes. The rotational and centrifugal distortion constants and the potential barrier height to methyl-top internal rotation, V 3 , were determined. The rotational constants were consistent with an Ar–DMS center of mass (cm) distance of 3.796 (3)Å and a S–cm–Ar angle of 104.8 (2)°. The V 3 potential barrier obtained, 736.17 (32)cm −1 , was 97.8% of the DMS monomer barrier. By assuming a Lennard–Jones-type potential, the dissociation energy was estimated to be 2.4kJ mol −1 , which was close to the value for Ar–DME, 2.5kJmol −1 .