A simple bent M←OL vibrational model has been used to estimate from ν(MO) and ν(SO) the force constants F MO and F OL for some O-bonded DMSO (dimethylsulfoxide), DMSO- d 6 and TMSO (tetramethylenesulfoxide) complexes. The correlation F OL = −(1.24 ± 0.2)F MO + (8.78 ± 0.12)mdyne/Å holds for all such complexes of the following metals in the oxidation states noted: Al III, Ca II, Cr III, Mn II, Fe II, Fe III, Co II, Ni II, Cu II, Ru II, Rh I, Rh III, Pd II, La III, Nd III, Yb III, Lu III, Pt II, Hg II, and is explained by a reduction of oxygen ( P π) → sulfur ( d π) double-bond character as the MO bond strengthens. The trend is usefully applied to aid the assignment of the i.r. spectra of a series of lanthanide(III)-DMSO complexes. The correlation does not hold for complexes of the group IVA and VA elements Sn IV, Sb III, Bi III and Bi V.