The antioxidant activity of capsaicin, as compared to BHT and melatonin, was determined by the direct measurement of lipid hydroperoxides formed upon linoleic acid autoxidation initiated by AIBN. The formation of four isomeric lipid hydroperoxides was detected after reverse-phase HPLC separation. Data from three detectors, UV absorption, glassy carbon electrode electrochemical detection, and postcolumn chemiluminescence using luminol, were compared. Capsaicin was more effective than melatonin in suppressing the formation of lipid hydroperoxides but not as effective as BHT. The formation of capsaicin and BHT dimers was observed during oxidation, and the dimers were characterized using APCI MS(n).