A molecular level description of the effect of urea on the stability of proteins or supramolecular aggregates such as micelles requires information about its interfacial concentration. Estimating interfacial urea concentrations has proved refractory because urea's association with interfaces is difficult to observe spectrometrically. Here we used a chemical trapping method based on product yields from spontaneous dediazoniation of micelle-bound 2,6-dimethyl-4-hexadecylbenzenediazonium ion to estimate the interfacial concentration of urea in micelles of cationic cetyltrimethylammonium halide (CTAX, X = Cl, Br), anionic sodium dodecyl sulfate, SDS, and sodium dodecyl sulfonate, SDSu, and zwitterionic N-hexadecyl-N,N-dimethylammonium propanesulfonate, HPS, surfactants. Up to 6-8 M added urea, the interfacial urea concentration is about 70% (HPS), 80% (CTABr), and 90%0 (CTACl, SDS, and SDSu) of its stoichiometric concentration in solution, that is, not much less than its concentration in bulk solution. These results are the first direct estimate of interfacial urea concentrations. We also obtained estimates of interfacial water and counterion concentrations in CTAX micelles and interfacial water and headgroup concentrations in SDSu micelles. Taken together, the results provide substantial evidence that urea destabilization of interfacial headgroup . counterion ion pairs is a major contributor to urea's effects on micellar properties such as increasing the critical micelle concentration and the degree of ionization and decreasing aggregation number.