Based on a previously discovered method for amine oxidations using flavins as catalysts and hydrogen peroxide as oxidant, a comparative kinetic study using NMR spectroscopy was undertaken with a series of flavins for amine and thioether oxidations. Included in this series is the newly prepared 7,8-difluoro-1,3-dimethyl-5-ethyl-5,10-dihydroalloxazine. This study shows that flavins, which bear electron-donating groups on the aromatic ring and/or the N-10 position, are less active and are deactivated during the course of the reaction. Moreover, flavins that are alkylated at the N-1 position instead of the N-10 position and having either no substituents or electron-withdrawing groups on the aromatic ring, remain the most active and stable.