Chitosan-poly(acrylic acid) polyelectrolyte complex nanoparticles were prepared by coacervation under mild experimental conditions Without the use of any organic solvents or surfactants. The influence of some experimental parameters such as the pH of the polyelectrolyte solutions, their concentrations, and the purification procedure on the particle dimensions and their size distribution was studied in detail. The physicochemical properties of the obtained complex were characterized with Fourier transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, and dynamic light scattering. It was found that for solution concentrations below 0.1 wt %, it was possible to obtain suspensions of nanometer-sized particles. Furthermore, it was established that the pH values of the reactant solutions had a great influence on both the particle size and the yield of the complex that was formed. The most convenient pH values for obtaining chitosan-poly(acrylic acid) particles with a nanometric size and optimum yield (near 90%) were found to be 4.5-5.5 for chitosan and 3.2 for poly(acrylic acid). Additionally, the effects of dialysis and ultrasonic treatment on the stability of complex suspensions, prepared under different experimental conditions, were clarified so that recommendations could be made to bring this system into practical use. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2362-2371, 2009