The dissociation of protonated peptides containing adjacent arginines has been studied by electrospray ionization-tandem mass spectrometry (ESI MS/MS) and theoretical calculations. The experimental results show that singly protonated peptides cleave at the Arg?Arg amide bond and generate the y₁ ion when adjacent arginines are the C-terminal residues. The major cleavage occurs at the C-terminal amide bond and produces the b_n-1 ion when adjacent arginines are not the C-terminal residues. The diketopiperazine and oxazolone fragmentation pathways of protonated NRR (Asn?Arg?Arg) have been investigated at the B3LYP/6-31G(d) and B3LYP/6-31++G(d,p) levels of theory. The geometries and energies of transition state species and hydrogen-bonding interaction are also discussed.