The base-catalyzed hydrolysis of flavins was investigated in 50 percent water-methanol with excess sodium hydroxide or excess amines at different temperatures. Rate constants for the pseudo first-order cleavage of the pyrimidine portion of the isoalloxazine systems were measured and with sodium hydroxide shown to increase from lumiflavin to 3-ethyllumiflavin to 3-carboxyamido(phenylalanyl)lumiflavin. With amines, the size as well as basicity is of prime importance since the general order in efficacy of the nucleophile for hydrolysis of flavins is ethylamine > diethylamine > triethylamine > benzylamine > tributylamine. In general, alkylation of the 3-imino function of the flavin increases its lability because of inability to ionize and undergo resonance stabilization of the anionic tautomers. Moreover, the special case where a 3-carboxyamido function is present allows intramolecular polarization, <i>via</i> hydrogen bonding of the amide hydrogen to 4-carbonyl oxygen, which causes a marked increase in hydrolysis rate in sodium hydroxide. From the temperature dependencies of the rates of hydrolysis, the calculated values for energies of activation indicate the decrease expected upon 3-alkylation and further upon intramolecular enhancement with the 3-carboxyamido group. The calculated values for entropies of activation are large and negative in accord with considerable loss of degrees of freedom in the activated complexes formed in the bimolecular mechanism.