Riboflavin synthase catalyzes the transfer of a four-carbon fragment between two molecules of the substrate, 6,7-dimethyl-8-ribityllumazine, resulting in the formation of riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H-pyrimidinedione. Earlier, a pentacyclic adduct formed from two substrate molecules was shown to be a catalytically competent intermediate, but the mechanism of its formation is still poorly understood. The present study shows that the recombinant N-terminal domain of riboflavin synthase from Escherichia coli interacts specifically with the exomethylene-type anion of 6,7-dimethyl-8-ribityllumazine but not with any of the tricyclic adduct-type anions that dominate the complex anion equilibrium in aqueous solution. Whereas these findings can be implemented into previously published mechanistic hypotheses, we also present a novel, hypothetical reaction sequence that starts with the transfer of a hydride ion from the 6,7-dimethyl-8-ribityllumazine exomethylene anion to an electroneutral 6,7-dimethyl-8-ribityllumazine molecule. The pair of dehydrolumazine and dihydrolumazine molecules resulting from this hydride transfer is proposed to undergo a 4 + 2 cycloaddition, affording the experimentally documented pentacyclic intermediate. In contrast to earlier mechanistic concepts requiring the participation of a nucleophilic agent, which is not supported by structural and mutagenesis data, the novel concept has no such requirement. Moreover, it requires fewer reaction steps and is consistent with all experimental data.