A number of spectrophotometric studies [Graminski, G. F., Kubo, Y. & Armstrong, R. N. (1989) Biochemistry 28, 3562–3568; Liu, S., Zhang, P., Ji, X., Johnson, W. W., Gilliland, G. L. & Armstrong, R. N. (1992) J. Biol. Chem. 267, 4296–4299] have recently shown that the glutathione (GSH) thiol is deprotonated when it is in complex with glutathione S-transferase. Different models have been proposed for the activation of the glutathione Sy, all pointing out the key role of activesite residue Tyr7. It remains unclear, however, how Tyr7 is actually involved in this process. In this paper we present an analysis of the electrostatic potential in the region of the active site of a π-class GSH transferase. This analysis provides evidence that the titration behaviour of the absorption band of the E · GSH complex with a pK between 6 and 7 [Liu, S., Zhang, P., Ji, X., Johnson, W. W., Gilliland, G. L. & Armstrong, R. N. (1992) J. Biol. Chem. 267, 4296–4299] should rather be explained by the protonation/deprotonation equilibrium of Tyr7 than by the protonation/deprotonation equilibrium of the GSH thiol group itself. On the basis of this conclusion, a mechanism for activation of GSH is proposed: the Tyr7 OH group is deprotonated by the influence of the protein charge constellation and the peptide dipoles. Thus it acts as a general base, promotes proton abstraction from the GSH thiol and creates a thiolate anion with high nucleophilic reactivity.