In the presence of a catalytic quantity of alkali metal mixtures of sulphur diimides RNSNR and R′NSNR′(R, R′ = Ph, 4-C6H4Me, 4-C6H4OMe, SiMe3, SPh) undergo a rapid scrambling of the R and R′ groups. When R and R′ are significantly different (e.g., R = Ph, R′ = SiMe3 or SPh) the equilibrium is shifted to favour the unsymmetrical sulphur dimmide (R ≠ R′); the procedure thus represents an effective method for preparing such derivatives. A mechanism involving the centrosymmetric association and rearrangement of two sulphur diimide radical anions is suggested for the observed ligand scrambling. The synthesis and X-ray structure determination of the mixed diimide (4-NO2C6H4)SNSN(4-C6H4OMe) is reported. The crystals are monoclinic, space group P21/n; the molecules stack as plates along the a axis in a head-to-head fashion, producing an interplanar spacing between consecutive S2N2 units of 3.421 Å. Key words: sulphur diimide radical anions, skeletal rearrangements, (4-C6H4NO2)SNSN(4-C6H4OMe), X-ray structure.