Abstract—Quenching of the excited states of lumiflavin and 3‐methyl‐5‐deazalumiflavin by methyl‐and methoxy‐substituted benzenes and naphthalenes in methanol was investigated. The observed difference in the reactivity of acid and neutral lumiflavin triplets is explained thermodynamically by applying the Michaelis cycle, as being due to the higher reduction potential of the acid triplet. In this connection the pKvalues of lumiflavin triplet (pKM= 6.5) and semiquinone (pKM= 11.3) have also been determined in methanol. The difference in the reactivity between the singlet and triplet states of lumiflavin is found to be greater as predicted by the difference in excitation energy. The reactivities of the excited states of flavin and 5‐deazaflavin differ only slightly in contrast to the marked difference in the ground state reactivities of electron transfer reactions. This is explained in terms of the model of Rehm and Weller. The pH dependence of the electron transfer quenching of 5‐deazaflavin triplet was investigated in water, yielding a triplet pKof 2.5. In contrast to the flavin, this triplet pKdoes not significantly differ from the pKof the 5‐deazaflavin ground state. From this, different sites of protonation are deduced for the photoexcited triplet states of flavin