The micellization and adsorbed film formation for an aqueous binary mixed system of anionic top-heavy type surfactants: sodium salt of a-sulfonatomyristic acid isopropyl ester (a-SMy iPr) and sodium 1-oxo-1 [4-(tridecafluorohexyl) phenyl]-2-hexanesulfonate (FC6-HC4), were studied by means of surface tension measurement (drop volume method). The surface tension (g) was precisely measured for the solutions of the respective single and eleven mixed systems at 30.00 0.03 in order to determine critical micellization concentrations (CMC) and surface excesses (G) as a function of net mole fraction of FC6-HC4 (X FC6-HC4). The effect of added NaCl on CMC was examined for each single system to determine the degree of counterion binding (b i) from the Corrin-Harkins plot. Using the data of CMC and b i , we estimated the micellar composition (Y FC6-HC4), activity coecients (f i) of the two surfactants in mixed micelles, and the interaction parameter (w 0), the calculation of which was made on the basis of our own theoretical equations taking into account of mutual counterion effect in binary ionic surfactant mixtures. From the CMC-Y FC6-HC4 curve, markedly positive and slightly negative deviations from ideal mixing micelle formation were found, i.e. the positive deviation at the range of 0 < X FC6-HC4 < ca. 0.3 and the slightly negative deviation at the range of ca. 0.3 < X FC6-HC4 < 1. On the other hand, the composition of adsorbed film (Z FC6-HC4) was also estimated as a function of X FC6-HC4 using the surface tension data from below CMCs. In addition, G was determined as functions of X FC6-HC4 and Z FC6-HC4 and the partial molecular area (PMA) was evaluated from the plot of mean molecular surface area (A m) vs. Z FC6-HC4. All the data obtained showed without exception that an addition of 10 to 20% a-SMy iPr to FC6-HC4 results in a marked enhancement of surface activity and micelle formation ability, accompanying a favorable packing in two-dimensional or three dimensional structuring by the two top-heavy type surfactants, and that the two or three dimensional structures are thermodynamically more stable due to higher FC6-HC4 content.