The mechanism of dediazoniation of 2,4,6-trimethylbenzenediazonium ion, 1-ArN2+, in concentrated aqueous solutions of acetamide, N-methylacetamide, and N,N-dimethylacetamide (peptide bond models) was probed by a combination of techniques including HPLC, GC/MS, and (H2O)-O-18 isotopic labeling. The kinetics and product distributions are completely consistent with the heterolytic dediazoniation mechanism, i.e., rate-determining loss of N-2 followed by trapping of the aryl cation intermediate, 1-Ar+, by H2O and the oxygens and nitrogens of the amides. Aryl imidates formed from trapping by amide O hydrolyze rapidly into aryl ester/amine and amide/phenol product pairs. The results were used to estimate the selectivity of 1-Ar+ toward the amide oxygens and nitrogens versus H2O. 1-Ar+ is only 10-40% more selective toward H2O than amide O, but it is more than 10 times mon selective toward H2O than the amide N. 1-Ar+ is slightly more selective toward the N of acetamide than N-methylacetamide. However, within the HPLC detection limit, 1-Ar+ does not give a product from reaction with the N,N-dimethylacetamide nitrogen. The selectivities are interpreted by using a preassociation model, i.e., selective solvation by the different nucleophiles of the reactive diazonio group in the ground state. These results indicate that chemical tagging (trapping by N) and cleaving (trapping by O) of the peptide bonds and the weakly basic side chains of polypeptides and proteins bound to association colloids, vesicles and biomembranes, and emulsions may provide new information on their topologies and orientations at the aggregates＇ interfaces.