Due to its strained five-membered ring, alpha-lipoic acid (LA) has an absorption band around 330 nm, which is used to quantify its concentration. In order to obtain information for the homolytic rupture of the S-S bond and the formation of dihydrolipoic acid (DHLA), the photochemical reaction of lipoic acid was examined in the presence or absence of ascorbic acid upon exposure to UVA light. The absorption band of alpha-lipoic acid at around 330 nm disappeared by photoirradiation, which corresponds to the rupture of S-S bond of the 1,2-dithiolane ring in lipoic acid. HPLC-Electrochemical Detection (ECD) analysis of lipoic acid showed significant formation of dihydrolipoic acid and other thiols. The formation of thiols from the photoreaction of lipoic acid was also confirmed by the Ellman method. The formation of thiols from lipoic acid was completely time-dependent and the formation of the thiols increased upto 55%. Similar results were obtained in the photochemical reactions of short-chain analogues, bisnor- and tetranor-lipoic acid. On the other hand, beta-lipoic acid was quite stable, no photodecomposition of beta-lipoic acid was observed in the UV region. The formation of thiols including dihydrolipoic acid from lipoic acid can be explained by considering the rupture of S-S bond, which results in the formation of the dithiyl radicals of alpha-lipoic acid. It is proposed that intra- and intermolecular hydrogen abstraction of the dithyl radical produces thiols including dihydrolipoic acid as final products.