Surfactants used for practical applications are usually surfactant mixtures because they often exhibit superior performance than the individual surfactants. In the present study binary mixtures of the sugar based n-dodecyl-β-D-maltoside (β-C₁₂G₂) with cationic dodecyltrimethyl-ammonium bromide (C₁₂TAB) and nonionic hexaethyleneglycol dodecyl ether (C₁₂E₆), respectively, have been investigated at different mole fractions. Surface tension measurements were used to determine the critical micelle concentration and the surfactant composition at the surface. Interaction parameters and the mole fractions of the individual surfactants in the mixed air/water monolayer as well as in the mixed micelles were calculated using regular solution theory. It was found that n-dodecyl-β-D-maltoside interacts weakly with the cationic surfactant (C₁₂TAB) and it dominates in both the mixed monolayer and the mixed micelles. On the other hand, β-C₁₂G₂ and C₁₂E₆ mix ideally in solution. For both surfactant mixtures the surfactant composition at the surface determined by surface tension measurements and by the regular solution theory, respectively, has been compared and discussed in detail.