A method has been elaborated for the large scale isolation of outer membranes of beef liver mitochondria. From these monoamine oxidase was partially purified and digested with trypsin and chymotrypsin to yield peptides containing riboflavin covalently linked to the peptide chain. A pure pentapeptide of riboflavin-5′-phosphate has been obtained by a series of chromatographic procedures. Edman degradation, followed by dansylation, revealed the amino acid sequence: Ser-Gly-Gly-X-Tyr, X being the amino acid to which the flavin is attached via the 8α-carbon of the riboflavin. This site of attachment is concluded on the basis of analysis of the optical spectra in the neutral and cationic forms of the flavoquinone and of the electron spin resonance spectrum of the free radical cation.