A surface-active arenediazonium ion, 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, was used as a chemical trapping probe of the concentrations of weakly basic nucleophiles in the surfactant film between the aqueous and oil domains of opaque relatively long-lived C12E5/octane/water macroemulsions as a function of temperature and added NaCl. The results demonstrate that the chemical trapping method "sees" the composition of the oligooxyethylene or interfacial layer on the aqueous side of the surfactant film. Product yields from trapping of the available nucleophiles, H2O, the terminal OH group of C12E5, and Cl-, were used to estimate average hydration numbers of all aggregates present in the macroemulsions as a function of increasing temperature and added NaCl. At 19.9 degreesC, the average hydration number in the macroemulsions is 2.5, compared to a 70% larger value of 4.2 in C12E6 micelles at 20 degreesC obtained previously by chemical trapping. Average hydration numbers decrease with increasing temperature and with added NaCl at constant temperature as macroemulsion structure changes from oil-in-water type through the balanced point to water-in-oil type. These results are consistent with the oriented wedge theory of macroemulsion stability. Chemical trapping estimates of Cl-concentrations within the interfacial layer of the surfactant film of the macroemulsions show that the molarity of Cl- in the aqueous region of the interfacial layer is always about 10% greater than the Cl- molarity in the aqueous domain. Thus, Cl-, and probably Na+, move freely between the bulk aqueous domain and the surfactant film of the macroemulsions, contradicting an assumption of adsorption/depletion model for the effect of lyotropic salts on the hydration of the interfacial layer. These results demonstrate the potential of chemical trapping for probing surfactant film compositions of opaque macroemulsions.