The interaction and synergism of some polyoxyethylenated fatty alcohol ether (POE) nonionic surfactants (C 12 E 2 , C 12 E 3 , C 10 E 5 , C 10 E 7 , where C x indicates number of carbon atoms in the chain and E y indicates number of oxyethylene glycol ethers) with trioxyethylenated dodecyl sulfonate (C 12 E 3 S) in mixed monolayer formation at the surface and in mixed micelle formation in aqueous solutions were studied at 25 and 40°C by calculating interaction parameters (β σ , β M ) from surface tension-concentration data by use of Rosen's equations based on the nonideal solution theory. All the systems investigated adapt reasonably well to the nonideal model, with negative values of β σ and β M (where M means micelle and σ refers to the air-liquid interface) indicating a favorable interaction between the mixed surfactants. Either at a monolayer or in a mixed micelle, the attractive interaction becomes stronger when the alkyl chain in the POE surfactant is longer, i.e., when the POE becomes more hydrophobic. The interaction increases in the order C 10 E 7 < C 10 E 5 < C 12 E 3 , C 12 E 2 . For the two C 10 E n (n = 5,7)/C 12 E 3 S systems, as temperature increases from 25 to 40°C, the interaction increases in a mixed micelle, but it decreases in a mixed monolayer. Synergism in mixed micelle formation exists for C 12 E 3 S/C 10 E n mixtures when X 1 M , the mole fraction of POE in a mixed micelle, is ~0.4-0.8, whereas synergism does not occur in the systems of C 12 E 3 S/C 12 E m due to the large difference between CMC 1 and CMC 2 , i.e., large ln(C 1 M /C 2 M ) value (where CMC = critical micelle concentration). The degree of synergism in mixed micelle formation is temperature independent and is 0.23, 0.18, and close to zero for C 10 E 5 /C 12 E 3 S, C 10 E 7 /C 12 E 3 S, and C 12 E m (m = 2,3)/C 12 E 3 S systems, respectively. Synergism in surface tension reduction effectiveness occurs in C 12 E 3 S/C 12 E 2 and C 12 E 3 S/C 12 E 3 systems. The mole fractions of POE in the solution phase are 0.302 and 0.333 for the two mixtures at the point of maximum synergism.