Cysteine is an infrequently occurring but important amino acid critical to tertiary and quaternary structure. While peptide chains typically undergo post-translational modifications, cysteine is susceptible to further modification due to its reactive thiol group. The effects of these modifications are unclear, as they could either represent normal cell-cell communication or potentially harmful by-products. Common agents that affect the thiol group of cysteine are by-products from other biological processes, for example metabolic processes (which creates H₂O₂), natural toxicant defense (p450 or glutathione), and inflammation (HOCl). This experiment sought to discern the products that form when cysteine is exposed to biologically relevant concentrations of these oxidants. Liquid chromatography-mass spectrometry (LC-MS) was used to perform the analysis of the oxidation products. Cysteine was mixed with 5 mM bleach in vitro and pH buffered and the resulting solution was analyzed. Methods were developed by optimizing instrumental conditions—including molarity of solutions, voltage spray and flow rate, and acidity of the samples—in order to use LC-MS. Pending results will allow for the identification and quantity of the cysteine oxidation products.