Dynamic light scattering, static light scattering, and turbidimetry were used to investigate the effect of polyelectrolyte molecular weight and concentration on complex formation between a strong polyelectrolyte, poly(dimethyldiallylammonium chloride) (PDMDAAC), and oppositely charged mixed micelles of Triton X-100 (TX100) and sodium dodecyl sulfate (SDS). The hydrodynamic radius (R(h)) of the complexes is about twice those of the PDMDAAC from which they are formed, while the radius of gyration (R(g)) of the complexes remains essentially unchanged. This suggests that the complex formed is compact. With increasing polyelectrolyte concentration, intrapolymer complexes transform to interpolymer complexes. The interpolymer complexes are not stable and in time phase separate (coacervate). Although the polyelectrolyte molecular weight has only a small effect on the structure of the complex in the intrapolymer complex region, the intra- to interpolymer complex transition depends strongly upon the molecular weight of the polyelectrolyte. The polyelectrolyte concentration at which the transition occurs decreases with increasing polyelectrolyte molecular weight. If the polyelectrolyte molecular weight is low enough, this sharp transition is not observed, and no associative phase separation is observed.