Two different 1 3 C-labeled 7-trifluoromethyl-N 1 ,N 1 0 -ethyleneisoalloxazinium chlorides were utilized to examine the mechanism of amine dehydrogenation. 1 H NMR studies in CD 3 CN ( 1 3 C NMR in DMSO-d 6 ), as confirmed using 1 5 N-labeled benzylamine, indicate that primary and secondary amines add to give tetrahedral C 1 0 a adducts that persist for hours at 25 o C. Upon heating, the C 1 0 a amine adducts partition between rearrangement to C 4 a spirohydantoin amidines, and, in the case of benzylic amines, β-elimination to give reduced flavin and imine dehydrogenation product. A C 1 0 a tetrahedral hydroxy adduct, generated under basic conditions when water was present, was also confirmed by 1 H/ 1 3 C NMR.