Reaction of hydriodic acid with peptides and proteins has been studied. At the boiling point, hydrolysis of the peptide bond, particularly stable bonds linking valine and isoleucine residues, is facile. Several amino acids react with constantboiling hydriodic acid but the only reactions detrimental to the amino acid analysis are the reduction of serine with concomitant formation of alanine, and the destruction of tryptophan. Gentler conditions of hydrolysis with diluted hydriodic acid are required for analysis of serine. Good results for analysis of proteins for amino acids may be obtained after a 6-hr hydrolysis period.