Complexes of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and N-dodecyl-methacrylamide (DodMAm) (poly(A/Dx), where x denotes the mol % content of DodNlAm) and n-dodecyl hexa(ethylene glycol) monoether (C12E6) exhibited a liquid-liquid phase separation (coacervation) when poly(A/Dx) with x approximate to 50 mol % and C12E6 were mixed in water in the presence of a sufficiently high concentration of added NaCl ([NaCl]). Phase diagrams obtained by turbidimetric titrations indicated that coacervation occurred only within a limited concentration regime both for the polymer and surfactant. Coacervation occurred more easily at higher [NaCl]; [NaCl] should be higher than ca. 0.2 M at 25 degreesC for coacervation to occur. The cloud points of a mixture of poly(A/D48) and C12E6 at varying [NaCl] suggested that the dominant effects of added NaCl were the shielding effect of electrostatic repulsion between sulfonate moieties in poly(A/D48) at lower [NaCl] and an enhancement of hydrophobic interaction caused by added salt at higher [NaCl]. From the intra- and interpolymer nonradiative energy transfer and quasielastic light scattering data, it was concluded that as the concentration of either of the three components (i.e., polymer, surfactant, and salt) is increased toward a phase boundary for coacervation, the size of poly(A/D48)-C12E6 soluble complexes, in which C12E6 micelles act as cross-linking junctions of different polymer chains, increases gradually at first and then abruptly near the phase boundary, ending up with coacervation at a higher concentration of the component. On the basis of these experimental data, we discussed a possible mechanism for the coacervation of poly(A/D48) with C12E6.