In this paper, oxidative damage to the cysteine (CySH) side-chain on a glassy carbon electrode (GCE) was investigated. Voltammetric studies show that there are three anodic peaks for the oxidation of CySH, which arise from (1) the oxidation of the -SH side-chain, forming cystine (0.71 V, vs. SCE) and (2) CySO( x )H, x = 2, 3 (0.98 V vs. SCE), and (3) the oxidation of the amino acid carboxyl group (around 1.51 V vs. SCE). The influence of dissolved oxygen, pH, scan rate, scan time, temperature and CySH concentration were investigated and the oxidative mechanism proposed. The peaks near 0.71 and 0.98 V are the promising candidates for measuring the oxidation of CySH on the GCE. This paper provides a new strategy for researching oxidative damage of amino acids, sulfur-containing peptides and proteins.