The reactivity of the cobalt-carbon bond towards sulphur dioxide has been investigated in the compounds RCoIII(L)B where L = N, N’-ethylenebis(salicy1- aldiminato), N, N’-phenylenebis(salicylaldiminato), or bis(dimethylg1yoximato); B = water or pyridine. In the Schiff base complexes, for R = alkyl, insertion. into the cobalt-carbon bond proceeds smoothly to give the corresponding sulphinato derivatives. Reaction also occurs with methylcobaloxime and a pentafluorobenzenecobalt- Schiff base compound, but the products are not so well defined. There is no reaction with cobalt-aryl bonds for any of the chelates. The alkane- and arenesulphinato complexes can be synthesized by an alternative oxidative-addition reaction NaCOL+RSO2Cl or a replacement reaction XCoL+AgSO2R or NaSO2R (X = C1, I). The infrared spectra of the complexes show that the sulphinato group is bonded in the manner shown: (Formula Presented) The p.m.r. spectra of the complexes are discussed in some detail. Reactions of the sulphinato derivatives with donor molecules and the halogens are briefly described.