Nonstoichiometric interpolyelectrolyte complexes (IPECs) as colloidal dispersions have been widely used for the past decade as reactive materials for flocculation and surface modification. In this context, some new aspects of the preparation and properties of IPEC nanoparticles based on NaPAMPS, in salt-free aqueous solutions, are reported in this article. IPEC dispersions with different characteristics, z-averaged particle sizes, polydispersity indices, and colloidal stabilities were tailored by the addition rate of the titrant, a less investigated factor in the synthesis of IPECs as nanoparticles. Poly(sodium 2-acrylamido-2-methylpropanesulfonate) (NaPAMPS) and two polycations bearing positive charges in the backbone, poly(diallyldimethylammonium chloride) and a polycation containing 95 mol % N,N-dimethyl-2-hydroxypropyleneammonium chloride units, were used for this purpose. The complex nanoparticle characteristics and storage stability were monitored via the optical density at 500 nm and dynamic light scattering. IPEC nanoparticles with z-averaged particle sizes of 100-250 nm resulted from the same polyion pair and the same polyion concentrations when the addition rate of the titrant, either the polyanion or polycation, varied within the range of 0.1-1.5 mL/mL of the starting polyion x h. (C) 2004 Wiley Periodicals, Inc.