A viscometric analysis for studying the polyelectrolyte complexation mechanism of sodium poly(styrenesulfonate) (NaPSS) by addition of diallyldimethylammonium chloride-acrylamide copolymers (DADMAC-AAM copolymers) was developed. The viscosity of the reacting mixture was studied as a function of the mixing ratio (ratio of the polycationic to polyanionic groups). A minimum viscosity (point of equivalence) was observed for an equimolar mixing ratio, thus indicating a 1:1 stoichiometry of the polyelectrolyte complexes (PECs) at this point. With an increase of the cationic charge density (35-100%) of the copolymer, the packing density of the complexes increased (8-60 mg/mL). For a charge density below 35% of the copolymer, flocculation occurred before the equimolar mixing ratio was reached. This was due to the larger size of the formed aggregates. To obtain additional structural information (stoichiometry, density) on the PECs before or after the point of equivalence, the contributions of the free polyelectrolyte in excess and of the PEC to the specific viscosity of the reacting mixture were estimated. Nonstoichiometric PECs were shown to be formed before the point of equivalence. Factors of stoichiometry (ratio of polyanionic to polycationic groups in the PECs) were evaluated to 1.4-1.6. The polyanionic excess tended to be lower at high cationic charge density and disappeared with increasing the mixing ratio until an equimolar ratio was reached. After the point of equivalence, the swelling of the PEC dominated.