Deamidation of Asn residues can occur either by direct hydrolysis of the Asn residue or via a cyclic imide intermediate. The effects of primary sequence on the pathways of deamidation of Asn residues were studied using Val-Tyr-X-Asn-Y-Ala hexapeptides with substitution on the C-terminal side (Y) and on the N-terminal side (X) of the Asn residue. In acidic media the peptides deamidate by direct hydrolysis of the Asn residue to yield only Asp peptides, whereas under neutral or alkaline conditions, the peptides deamidate by formation of the cyclic imide intermediates which hydrolyze to yield both isoAsp and Asp peptides. At neutral to alkaline pH's the rate of deamidation was significantly affected by the size of the amino acid on the C-terminal side of the Asn residue. The amino acid on the C-terminal side of the Asn residue has no effect on the rate of deamidation at acidic pH. Changes in the structure of the amino acid on the N-terminal side of the Asn residue had no significant effect on the rate of deamidation at all the pH's studied. For peptides that underwent deamidation slowly, a reaction involving the attack of the Asn side chain on the peptide carbonyl carbon resulting in peptide bond cleavage was also observed.