S-Nitrosoglutathione (GSNO) is a very important biomolecule that has crucial functions in many physiological and physiopathological processes. GSNO acts as NO donor and is a candidate for future medicines. This work describes, for the first time, the separation and the detection of GSNO and its decomposition products using capillary electrophoresis coupled to mass spectrometry (CE-MS). The separation was performed in slightly alkaline medium (pH 8.5) under positive-ionization MS detection. The identification of three byproducts of GSNO was formally performed for the first time: oxidized glutathione (GSSG), glutathione sulfinic acid (GSO2H), and glutathione sulfonic acid (GSO3H). GSO2H and GSO3H are known to have important biological activity, including inhibition of the glutathione transferase family of enzymes which are responsible for the elimination of many mutagenic, carcinogenic, and pharmacologically active molecules. We observed, after the ageing of GSNO in the solid state, that the proportion of both GSSG and GSO3H increases whereas that of GSO2H decreases. These results enabled us to propose an oxidation scheme explaining the formation of such products.