The formation and characterization of some interpolyelectrolyte complex (IPEC) nanoparticles based on poly(sodium 2-acrylamido-2-methylpropanesulfonate) (NaPAMPS), as a function of the polycation structure, polyanion molar mass, and polyion concentration, were followed in this work. Poly(diallyldimethylammonium chloride) and two polycations (PCs) containing (N,N-dimethyl-2-hydroxypropyleneammonium chloride) units in the backbone (PCA(5) and PCA(5)D(1)) were used as starting polyions. The complex stoichiometry, (n(-)/n(+))(iso), was pointed out by optical density at 500 nm (OD500), polyelectrolyte titration, and dynamic light scattering. IPEC nanoparticle sizes were influenced by the polycation structure and polyanion molar mass only before the complex stoichiometry, which was higher for the more hydrophilic polycations (PCA(5) and PCA(5)D(1)) and for a higher NaPAMPS molar mass, and were almost independent of these factors after that, at a flow rate of the added polyion of about 0.28 mL x (mL PC)(-1) x h(-1). The IPEC nanoparticle sizes remained almost constant for more than 2 weeks, both before and after the complex stoichiometry, at low concentrations of polyions. NIPECs as stable colloidal dispersions with positive charges in excess were prepared at a ratio between charges (n(-)/n(+)) of 0.7, and their storage colloidal stability, as a function of the polycation structure and polyion concentration (from 0.8 to ca. 7.8 mmol/L), was demonstrated. (C) 2004 Wiley Periodicals, Inc.