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 bn+/[bn-1+cat]+ species. In this study the influence of a 4-aminomethylbenzoic acid (4AMBz) residue on the relative intensities of [b(3)-1+cat]+ and [b(3)+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 [b(3)-1+cat]+ for the peptides that contain the 4AMBz residue, and in some cases the complete elimination of the [b(3)+17+cat]+ pathway. The influence of the 4AMBz residue may be attributed to the fact that [b(3)-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 [b(3)-1+Na]+ relative to that of [b(3)+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.