Abstract: The reactions between cobalt xanthates, CoA_3(A=RXan), and dimethylamine are investigated by means of high-performance liquid chromatography(HPLC). The reaction mixtures, containing CoA_3, CoA_2B, CoAB_2, and CoB_3(B=dimethyl dithiocarbamate), are separated on a Zorbax-sil column by mixed eluent of n-hexane and isopropanol (90:10). The retention times of the complexes are found to be increasing with the increase of the number of dimethyldithiocarbamate ligand in the complexes, i.e. CoB_>CoB_2A>CoBA_2>CoA_3(Fig.1, Fig.2). Further HPLC study shows that the reaction involve three steps of substitutions. The -OR groups are replaced one after another by-NMe_2 group (Fig.3). A new reacton product, dimethylthiourethane, is also discovered. Kinetic study on the first step of substitution in n-hexane reveals that the reactions are first order to cobalt complexes and 3rd order to dimethylamine (Fig.4, Table 1). The rate constants at 298 K for the complexes increase in the series, Co(EtXan)_3, Co(MeXan)_3, Co(BzXan)_3. The reaction mechanism is discussed and the present results favour the nucleophilic substitution prediction.