The thermal degradation of 7, 8-dimethyl-10-formylmethylisoalloxazine (FMF) in acid solution under anaerobic conditions has been studied. A specific spectrophotometric method has been used to determine FMF in the presence of degradation products. FMF has been found to undergo second-order reaction in the dark to form lumichrome and side chain products. The second-order rate constants for the reactions at 40-60 oC are in the range of 1.05 – 4.40 L mole-1 min-1. The activation energy and the frequency factor for the reaction have been calculated as 15.0 kcal/mole (62.8 kJ/mole) and 2.43 x 1010 L mole-1 min-1, respectively. A reaction scheme for the bimolecular degradation of FMF has been suggested.