High signal intensities of glutathione (GSH), [GSH+H]+ (m/z 308), cysteine (CySH), [CySH+H]+ (m/z 122), and homocysteine (hCySH), [hCySH+H]+ (m/z 136), are observed in ESI MS with on-line electrochemistry (EC). Dimers formed by H-bonding, which are not electrochemical products, are detected as [2GSH+H]+ (m/z 615), [2CySH+H]+ (m/z 243) and [2hCySH+H]+ (m/z 271) together with disulfide dimers GSSG, CySSCy and hCySSCyh, [GSSG+H]+ (m/z 613), [CySSCy+H]+ (m/z 241) and [hCySSCyh+H]+ (m/z 269). When dopamine is present a thiol/dopamine quinone (DAQ) adduct is observed. Formation of this adduct is proposed to occur by an electrochemical mechanism during ESI. Catalysis of thiol oxidation and analysis of thiol mixtures is addressed.