Products from spontaneous reaction of a short-chain, water soluble arenediazonium salt, 2,4,6-trimethylbenzenediazonium tetrafluoroborate (1-ArN2BF4), in aqueous micellar solutions of cetyltrimethylammonium halides ((CTA)X (X = Cl, Pr)) are used to estimate the degree of ionization, alpha, and the ion exchange constant, K-Br/Cl The arenediazonium ion (1-ArN2+) reacts by rate-determining loss of N-2 to give an aryl cation that traps available nucleophiles, i.e. H2O, Cl-, and Br-, to give stable phenol (1-ArOH) and halobenzene products (1-ArCl and 1-ArBr), respectively. Product yields are determined by HPLC from calibration curves obtained from independently prepared standards. Reproducible yields are obtained at very low halide ion concentrations, on the order of millimolar, well within the range needed to detect the "free counterions" in the aqueous intermicellar pseudophase. The basic assumption of the method is that 1-ArN2+ remains in the aqueous pseudophase at all (CTA)X and NaX concentrations. Trends in the Stern-Volmer constant for fluorescence quenching of Ru(bpy)(3)(2+) by 1-ArN2+ in (CTA)Cl/NaCl solutions strongly support this assumption. The results obtained by this method are in good agreement with literature values : alpha = 0.25 and 0.29 for (CTA)Br and (CTA)Cl, respectively, and K-Br/Cl = 2.65 +/- 0.4. Potential applications of the method are briefly discussed.