Novel cationic hydroxyethyl cellulose (HEC) polymers with different molecular weights (1.1 × 105 to 1.7 × 106 g/mol) and ethylene oxide (EO) side chain lengths (1.5?2.9 EO units) were mixed with sodium dodecyl sulfate (SDS) in aqueous solutions. The phase diagrams of cationic HEC?SDS complexes were determined in the dilute polymer concentration regime (<0.5 wt %) with gradual addition of SDS molecules. The viscosity and structures of the complexes during the phase evolution were studied using rheometry and dynamic light scattering. The gradual addition of SDS first induced interchain associations with the bound SDS aggregates serving as cross-linkers to form an open network structure, producing a very broad size distribution and high viscosities of the complex solutions, and then condensed the network and induced a structure reorganization, resulting in globular aggregates with narrow size distributions. The growth of these globular aggregates in size eventually led to macroscopic sedimentation near charge neutralization. Further addition of SDS randomly broke the sedimentary aggregates into small particles and SDS micelles with low solution viscosities. The effects of molecular weight and EO side chain length of polymers on the phase boundary, viscosity, and structure of cationic HEC?SDS complexes were discussed.