In the field of biological applications, polyelectrolyte complexes are proposed to encapsulate bioactive compounds, to deliver drugs, and also to transfect genes into cells under the name of polyplexes. Complex formation is obtained by addition of a polycation solution into a polyanion solution or vice-versa. This work proposes a theoretical approach to describe complex formation in the case of non-stoichiometric mixtures of oppositely charged macroions having different degrees of ionization and different degrees of polymerization under different salt conditions. In a second part, comparison was made with experimental data collected when a weak polybase, namely poly(L-lysine) under its bromide form was added stepwise to solutions of various polyanions under their sodium salt form, namely poly(L-lysine citramide imide), poly(L-lysine citramide), and poly(beta-malic acid), the latter lacking hydroxyl groups attached to the main chain. The stability of stroichiometric complexes made of poly(L-lysine) and poly(L-lysine citramide) having different molecular masses is discussed. (C) 2016 Wiley Periodicals, Inc.