As a powerful natural antioxidant, lipoic acid (LipSS) and its reduced form dihydrolipoic acid (DHLA) exert significant antioxidant activities in vivo and in vitro by deactivation of reactive oxygen and nitrogen species (ROS and RNS). In this study the riboflavin (RF, vitamin B2) sensitized UVA and visible-light irradiation of LipSS and DHLA was studied employing continuous irradiation, fluorescence spectroscopy, and laser flash photolysis (LFP). Our results indicate that LipSS and DHLA quench both the singlet state (1RF*) and the triplet state (3RF*) of RF by electron transfer to produce the riboflavin semiquinone radical (RFH.) and the radical cation of LipSS and DHLA, respectively. The radical cation of DHLA is rapidly deprotonated twice to yield a reducing species, the radical anion of LipSS (LipSS.-). When D2O was used as solvent, it was confirmed that the reaction of LipSS with 3RF* consists of a simple electron-transfer step, while loss of hydrogen occurs in the case of DHLA oxidation. Due to the strong absorption of RFH. and the riboflavin ground state, the absorption of the radical cation and the radical anion of LipSS can not be observed directly by LFP. N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and N,N,N',N'-tetramethyl benzidine (TMB) were added as probes to the system. In the case of LipSS, the addition resulted in the formation of the radical cation of TMPD or TMB by quenching of the LipSS radical cation. If DHLA is the reducing substrate, no formation of probe radical cation is observed. This confirms that LipSS.- is produced by riboflavin photosensitization, and that there is no oxidizing species produced after DHLA oxidization.