Thiolato-bridged dimolybdenum complexes [Mo2(CO)3(µ-SR)3(η-C7H7)] are formed by treating [Mol(CO)2(η-C7H7)] with RSH (R = Me, Et, Pri, or But) in the presence of NEt3. Infrared and variable-temperature n.m.r. spectroscopy reveal that the complexes exist as two isomers, arising from different relative orientations of the R groups on bridging sulphur. At ambient temperatures and above, these isomers interconvert rapidly, attributed to a low-energy sulphur inversion process, such that complete averaging of R group environments is achieved on the n.m.r. time scale. Intercoverting isomers are also present for the monosubstituted derivatives [Mo2(CO)2(PR′3)(µ-SBut)3(η-C7H7)][R′3= P(OMe)3, PMePh2, PMe2Ph, or PMe3], but for these only a limited averaging is observed, generating a time-averaged mirror plane of molecular symmetry. This results from the presence of a phosphorus ligand trans to one bridging sulphur and of CO trans to the other two, shown in an X-ray diffraction study of [Mo2(CO)2{P(OMe)3}(µ-SBut)3(η-C7H7)]. Crystals are monoclinic, space group P21/n, with four molecules in a unit cell of dimensions a= 10.186(7), b= 15.677(7), c= 19.821(15)A, and β= 96.31 (5)°. The structure has been solved by conventional techniques and refined by least squares to R 0.046 for 5 149 diffractometer-measured reflection intensities. Three bridging SBut groups display an asymmetric stereochemistry about a Mo–Mo bond [2.9460(15)A], rationalised on the basis of intramolecular terminal ligand–bridging ligand contacts. One molybdenum is η7 co-ordinated to a cycloheptatrienyl ring while the other carries the two CO groups and a P(OMe)3 ligand. Metal–sulphur distances range over 2.411(2)–2.549(2)A and Mo–P is 2.445(2)A. Free energies of activation for sulphur inversion as low as 46kJ mol–1 have been derived from coalescence temperatures for several complexes.