Formation of [bn + 17 + cat]+ is a prominent collision-induced dissociation (CID) pathway for Li+- and Na+-cationized peptides. Dissociation of protonated and Ag+-cationized peptides instead favors formation of the rival b/[bn−1 + cat]+ species. In this study the influence of a 4-aminomethylbenzoic acid (4AMBz) residue on the relative intensities of [b3−1 + cat]+ and [b3 + 17 + cat]+ fragment ions was investigated using several model tetrapeptides including those with the general formula A(4AMBz)AX and A(4AMBz)GX (where X = G, A, V). For Li+- and Na+-cationized versions of the peptides there was a significant increase in the intensity of [b3−1 + cat]+ for the peptides that contain the 4AMBz residue, and in some cases the complete elimination of the [b3 + 17 + cat]+ pathway. The influence of the 4AMBz residue may be attributed to the fact that [b3−1 + cat]+ would be a highly conjugated species containing an aromatic ring substituent. Comparison of CID profiles generated from Na+-cationized AAGV and A(4AMBz)GV suggests an apparent decrease in the critical energy for generation of [b3−1 + Na]+ relative to that of [b3 + 17 + Na]+ when the aromatic amino acid occupies a position such that it leads to the formation of the highly conjugated oxazolinone, thus leading to an increase in formation rate for the former compared to the latter. Copyright © 2007 John Wiley & Sons, Ltd.