The product of photolysis of K 3 (+)-[Co(l-cysi-N,S) 3 ] (1, l-cysi=l-cysteinesulfinate(2-) ion, cysi=NH 2 CH(COO - )CH 2 SO - 2 ) was separated by column chromatography on DEAE-Sephadex A-25. The main band contained [Co(l-cysi-N,S) 2 (l-cysi-N,O)] 3 - (2); linkage isomerization from sulfinato-S to sulfinato-O took place. This complex (2) is thermally unstable and reverts to the starting complex (1). An anion-exchange high-performance liquid chromatographic (HPLC) method was successfully applied to the kinetic study of this linkage isomerization reaction from 2 to 1.