Thiocyanate hydrolase (SCNase) is a member of a family of nitrile hydratase proteins, each of which contains a unique noncorrin cobalt center with two post-translationally modified cysteine ligands, cysteine-sulfenic acid or -sulfenate (Cys-SO(H)), and cysteine-sulfininate (CYS-SO2-), respectively. We have found that a partially matured recombinant SCNase was activated during storage The crystal structures of SCNase before and after storage demonstrated that CYS-SO2 modification of gamma CyS131 proceeded to completion prior to storage, while Cys-SO(H) modification of gamma Cys133 occurred during storage SCNase activity was suppressed when gamma Cys133 was further oxidized to CYS-SO2-. The correlation between the catalytic activity and the extent of the gamma Cys133 modification indicates that the cysteine sulfenic acid modification of gamma Cys133 is of primary importance in determining the activity of SCNase.