The counterion binding parameters (beta) of aqueous hexadecyltrimethylammonium acetate (C(16)TAOAc) are estimated from pH data at various C(16)TAOAc-AcOH concentrations with or without added sodium acetate buffer. In contrast to what is usually reported for ionic amphiphiles associated with less hydrophilic counterions, beta varies from ca. 0.4 to ca. 0.7 and depends upon the acetate ion concentration in the aqueous phase. The critical micelle concentrations (cmc) for C(16)TAOAc, C(16)TAOAc-NaOAc, C(16)TAOAc-AcOH, and C(16)TAOAc-NaOAc-AcOH solutions, measured from extinction data for fluorescence of acridine, are also examined in terms of AcO- concentration dependence. The plot of the logarithm of the amphiphile activity at cmc vs the logarithm of acetate ion activity is markedly curved, as expected for significant variations in beta.