Dieosinediglutathione is the product of glutathione disulfide and eosin isothiocyanate which has been purified and lyophilized. This glutathione disulfide derivative has low fluorescence due to self-quenching. The monomer eosinglutathione (EGSH) has high fluorescence. This product can be used for analysis of glutathionylation and deglutathionylation of proteins.