Viscometry, UV spectroscopy, and analytical ultracentrifugation (AUC) were applied to study preferential binding of poly(diallyldimethylammonium chloride) added to mixtures of sodium poly(styrenesulfonates) (PSS) of quite different molecular weights and PSS/sodium poly(methacrylate) (PMA). In salt-free solutions the decrease of the relative viscosity during complex formation was compared with model systems containing amounts of the free PSS components and NaCl, corresponding to special cases of preferential binding. The analysis suggests some preference for binding of the shorter polyanions. These findings were supported by AUC measurements of the composition of supernatant solutions after spinning down the polyelectrolyte complexes (PECs). In the presence of low molecular salt, the systems exhibit even binding at first (at ionic strengths of about 5 x 10(-3) N NaCl) and at higher salt contents a pronounced binding of the high molecular component. Even an exchange low molecular weight (LMW) PSS for high molecular weight (HMW) PSS was found in long-term experiments at higher ionic strength. In the case of polyanions of different natures, only a weak preference of PSS was observed by AUC and UV spectroscopy in salt-free media, whereas in the presence of NaCl, PSS molecules were strongly favored. PSS was also demonstrated to substitute PMA in the complexes at I > 7.5 x 10(-3) N NaCl. The peculiarities of preferential binding in salt-free solutions can be explained by thermodynamic nonequilibrium conditions.