Analytical ultracentrifugation was successfully applied to the determination of the complex composition and characterization of the particles of water insoluble polyelectrolyte complexes between sodium poly(styrenesulfonate) (PSS) and copolymers of various ratios of diallyldimetylammonium chloride (DADMAC) and acrylamide (AA). Sedimentation velocity experiments in an analytical ultracentrifuge with absorption optics allow the observation of individual species in a system of strongly polydisperse, aggregated particles of colloidal level in the presence of free polymer. It was found that the complexes are remarkably polydisperse and form nonstoichiometric particles including the major component in considerable excess. The stoichiometry factor depends strongly on the ionic strength of the solution and the concentration of the components. At low initial mixing ratios and ionic strengths of about 10(-2)-10(-3) N, sedimentation equilibrium experiments can be performed. The results of the evaluation of free PSS and complex concentration gradients using the Omega-function are similar to the findings obtained with sedimentation velocity experiments. A model where secondary aggregates are surrounded by free polymers is discussed for the sedimentation of these particles. It is demonstrated that polyelectrolyte complexes with the copolymer containing 50% DADMAC and AA form comparatively monodisperse species in aqueous salt solution having sedimentation coefficients between 4 and 40 Sv.