Irradiation of α-lipoic acid dissolved in various aliphatic aldehydes (molar ratio 1 : 200 or 1000,respectively) with an ultraviolet lamp gave the following results : (1) A considerable part of α-lipoic acid was converted to β-lipoic acid. (2) Trimerization of aldehyde markedly took place only when aldehyde was irradiated in the presence of α-lipoic acid. (3) In the irradiated mixture of α-lipoic acid and an aldehyde with branched carbon skeleton, such as isobutyraldehyde, an unknown lipoic acid-active substance was detected. The results of elementary analysis, infrared spectrophotometry and other tests have postulated the structure as I. The trimerization of aldehyde as well as the formation of the lipoic acid-active substance mentioned above seems to be initiated by the formation of an energy transfer complex of activated aldehyde with α-lipoic acid or a charge transfer complex of lipoic acid-biradical or -ion radical with aldehyde.