Mixed micellization and mixed adsorbed film formation were investigated for the combination of a Gemini type cationic and a nonionic surfactants mixture: Bis-trimethyl ammonium Gemini derived from tartaric acid bromide (BAGTB) and n-Decanoyl-Nmethylglucamide (MEGA-10). The surface tension of the aqueous mixed surfactant solution was measured at every 0.1 mole fraction of MEGA10 in the surfactant mixture applying a drop volume method at 30. From the curves of surface tension (g) vs logarithmic concentration in molality (ln m), critical micellization concentration (CMC), minimum surface tension at CMC (g CMC), surface excess (G), mean surface area occupied by a molecule (A m) and parameters related to synergism in surface activity such as pC 20 and CMC / C 20 were determined. Based on the regular solution theory, the relation of compositions of the singly dispersed phase (X MEGA10) and the micellar phase (Y MEGA10), and the relation of X MEGA10 with the composition in adsorbed film phase (Z 2) were estimated, and along with these, the interaction parameters in micelles (w R) and in adsorbed film (w A) were calculated. Both the CMC-X MEGA-10 and CMC-Y MEGA-10 curves showed a negative deviation from ideal mixing and even the curve of Z MEGA10-m t (bulk phase concentration) produced a slightly negative w A. However, the synergism in surface tension reduction was found to be rather weak from examination of partial molecular area (PMA) and the minimum free energy at surface G min (S) = (g CMC A m L). As for the adsorbed film, the interaction mode between molecules, as well as two dimensional molecular packing, was observed to be separated into three regions; i. e. at X STDS = 0.45 and at X STDS = 0.75 different properties changed discontinuously.