The glutathione S-transferase (GST) activities of S-type and L-type thioltransferases (TTases), which are purified from the seeds and leaves of Arabidopsis thaliana, respectively, were identified and compared. The S-type and L-type TTaes showed Km values of 9.72 mM and 3.18 mM on 1-chloro-2,4-dinitrobenzene (CDNB), respectively, indicating the L-type TTase has higher affinity for CDNB. The GST activity of the L-type TTase was rapidly inactivated after being heated at 70℃ or higher. The GST activity of the S-type TThse remains active in a range of 30-90℃. Hg^(2+) inhibited the GST activity of the S-type TTase, whereas Ca^(2+) and Ca^(2+) inhibited the GST activity of the L-type TTase. Our results suggest that the GST activities of two TTases of Arabidopsis thaliaraa may have different catalytic mechanisms. The importance of the co-existence of Vase and GST activities in one protein remains to be elucidated.