Abstract—Flash photolysis at 450 nm has been used to study the quenching of the excited triplet state of lumiflavin and the transient species formed in subsequent reactions in deaerated phosphate buffer (pH 6.9).The effect of the presence of ferricyanide on the life time of triplet lumiflavin has been studied. The results suggest an energy transfer reaction without concurrent electron transfer reactions. The rate constant for the process was 2.8 times 109M‐1s‐1. The analogous reaction with ferrocyanide could not be observed because of the efficient electron transfer reaction (δG = ‐20.6 kcal mol‐1) leading to the formation of the semireduced lumiflavin and ferricyanide. The rate constant for this reaction was 3.3 times 109M‐1s‐1. The semireduced lumiflavin radical was found to disappear in a second order reaction with a rate constant of 1.7 times 109M‐1s‐1. It was found to react with ferricyanide with a rate constant of 0.7 times 109M‐1s‐1.A model for the various photochemical and photophysical processes involved in the decay and quenching of the lumiflavin triplet state is s