GSSG/GSH Quantification Kit contains the reagent for GSH masking. GSH in a sample solution can be removed by adding the Masking reagent. Therefore, GSSG in the sample solution can be selectively determined by measuring the absorption (Lambda maxmax = 412 nm) derived from a colorimetric reaction of DTNB (5,5'-dithiobis (2-nitrobenzoic acid)) coupled with the enzymatic recycling system. The quantity of GSH can also be determined by subtracting the amount of GSSG from the total amount of glutathione.