Turbidimetry, dynamic light scattering, and capillary electrophoresis were used to study the complexation of polydiallyldimethylammonium chloride (PDADMAC) with mixed micelles of sodium dodecyl sulfate (SDS) and dodecanol ethoxylates (C(12)E(n)). The effect of EO chain length and its distribution was examined using various combinations of C(12)E(n) (n = 4, 6, 8, 12). The results show that the onset of the complexation of PDADMAC with the mixed micelles is affected by the EO chain length of C(12)E(n) : the mole fraction (Y) of SDS in the mixed micelle required to form the complex (Y-c) increases with n. The effect of EO chain length on the onset of bulk phase separation shows the same trend. Although Y-c varies with n, the electrophoretic mobility of mixed micelles with composition corresponding to Y-c is independent of n. We propose that the effect of EO chain length has two aspects : (1) an increase in the average distance between bound polycation segments and the SDS sulfate groups, and (2) an increase in the distance between SDS head groups, which causes a decrease in the surface charge density (sigma) of the micelle. Therefore, the electrical potential at the mean locus of bound polymer segments, psi(0), decreases with increasing n; in order for complexation to occur, this effect must be compensated for by a larger value of Y. Broader distributions of EO chain length lead to an increase in the range of Y over which the soluble complex is stable. We suggest that polycations initially bind to micelles which are rich in shorter EO chains and thus have higher "surface" potential, psi(0). However, additional SDS may go preferentially into micelles rich in longer chains with lower psi(0). This delays the formation of micelles which have sufficiently large psi(0) to cause phase separation.