The total-reflection XAFS measurement possessing bromide ion selectivity at the interfacial region was applied to the adsorbed film of hexadecyltrimethylammonium chloride (HTAC) and dodecyltrimethylammonium bromide (DTAB) mixture. The surface compositions X-j(H) of individual ions j (j = HTA(+), Cl-, DTA(+), and Br-) were evaluated by combining the surface excess concentration of Br- estimated from the XAFS with the surface composition of the respective surfactants from the surface tension results. It is clearly shown that HTA+ and Br- are preferentially adsorbed to DTA(+) and Cl-at the air/water interface. The preferential adsorption was estimated numerically in terms of activity coefficient f(i +/-)(H,p) of component i and excess Gibbs energy of adsorption (g) over cap (H,E)(pr). Then, the magnitude (g) over cap (H,E)(pr) was compared with that (g) over cap in(H,E) attributable to intrinsic interaction between ions.