Abstract The i.r. and Raman spectra of tetramethyldiphosphine disulphide (tmps) and of tmps-d 12 and tmps- 13C 4 are reported and the assignment of the skeletal modes is discussed. Raman polarization data confirm previous assignments for the PS and PP stretching modes and the symmetric PPS bend is assigned at Δν = 170 cm −1. There is no evidence of trans-gauche or trans-cis isomerism in solution: it is estimated that the molecule remains at least 99.5% in or very close to the trans form in chloroform or methylene chloride. The small dipole moment in benzene solution most probably arises from small distortions of the trans-structure, rather than trans-gauche isomerism. It has been suggested that the solid compound contains two significantly different types of tmps molecule but there is no evidence of this in the solid vibrational spectrum. Interactions between the skeletal and internal methyl vibrations are significant and are illustrated by the appreciable frequency shifts which occur in many of the skeletal modes in the deuterated compound.