The interaction between two hydrophilic polyelectrolytes of opposite charges was investigated using poly(L-lysine) (PLL) as the polycation and a library of copolymers of acrylamide and 2-acrylamido-2-methyl-1-propanesulfonate (P (AM-co-AMPS)) with various chemical charge densities as polyanions. The formation of polyelectrolyte complexes (PECs) was comparatively studied by varying different parameters, such as the mixing order, the P(AM-co-AMPS) chemical charge density and the initial polycation to polyanion molar ratio. PECs were then characterized in terms of charge stoichiometry and of stability toward ionic strength. The results showed a strong dependency of precipitated PEC stoichiometry on the P(AM-co-AMPS) chemical charge density and the initial polycation to polyanion molar ratio. In contrast, PEC stoichiometry was not affected by the mixing order of the two polyelectrolyte partners. A general rule capable of predicting the PEC stoichiometry is proposed.