Glutathionylation is the reversible redox modification of protein thiols by disulphide formation with glutathione. Glutathionylation can alter protein structure and activity in response to changes in the oxidation state of the protein, thus modulating protein stability¹. The forward reaction is largely spontaneous while the reverse reaction (deglutathionylation) is predominantly catalysed by the Glutaredoxin (Grx) family of thioltransferases¹. Glutathione transferase Omega 1 (GSTO1-1) is an atypical glutathione transferase that has minimal functional resemblance with other members of the superfamily. GSTO1-1 has previously been shown to have high thioltransferase activity like glutaredoxins^2,3  . This study investigates the role of GSTO1-1 in the glutathionylation cycle. Here, we show that human GSTO1-1, with a unique conserved cysteine at its active site, can catalyse the deglutathionylation of protein thiols in vitro and in cell lines. The kinetics of the catalytic activity of GSTO1-1 were determined in vitro by assaying the deglutathionylation of a synthetic peptide by tryptophan fluorescence quenching and in cell lines by means of immunoblotting and immunoprecipitation. We generated stable GSTO1-1 transfectants in T47-D breast cancer cells which are devoid of endogenous GSTO1-1. The over-expression of GSTO1-1 in these cells resulted in a global abatement of protein glutathionylation. Furthermore, we demonstrated that a mutation in the active cysteine residue (Cys-32) ablates the deglutathionylating activity, confirming the role of GSTO1-1 as a redox switch in regulating protein post translational modification. Additionally, we have developed and employed a novel and rapid method to quantify global glutathionylation in vivo to confirm the catalytic role of GSTO1-1 in the glutathionylation cycle⁴  . The study introduces GSTO1-1 as a novel member of the class of deglutathionylating enzymes which is currently restricted to glutaredoxins and sulfiredoxins.