A polyelectrolyte nanogel (PENG) particle consisting of lightly cross-linked terpolymer chains of N-isopropylacrylamide, acrylic acid, and 1-vinylimidazole has positive charges in an aqueous medium at pH 3 due to protonation of the imidazole groups, and thereby forms a polyelectrolyte complex with the linear polyanion, potassium poly(vinyl alcohol) sulfate (KPVS). It has been demonstrated that the hydrodynamic radius (Rh), by dynamic light scattering (DLS), and the radius of gyration (Rg), by static light scattering (SLS), of the complex particles are smallest at approximately 1:1 mixing ratio (rm) of anions to cations, in the absence of simple salts such as KCl (Langmuir 2005, 21, 4830). Here, we aimed to study the nature of the complex formed at rm=1 and examined the complex formation process by electrophoretic light scattering (ELS). It was found that the mobility of the cationic PENG with a stoichiometric amount of bound KPVS anions (i.e., the complex formed at rm=1) is positive but not zero at 25 degrees C. This was also the case when the complex was examined by ELS at 45 degrees C, where DLS and SLS show a temperature-driven collapse of the complex. We thus assumed that (a) electroneutrality is maintained in the complex particle with the aid of counterions, but (b) the complex is highly polarizable, and hence (c) during ELS the KPVS anions would dissociate in part from the complex. This hypothesis was supported by the following results: (i) Mixing complexed and uncomplexed PENG particles at different ratios brings about an increase in Rh and a decrease in the light scattering intensity of the complex at the same time, suggesting a polyelectrolyte exchange reaction. (ii) The same phenomenon is seen when poly(diallyldimethylammonium chloride) (PDDA as a polysalt) is added to the complex dispersion, meaning that the PDDA takes out the KPVS from the complex to form a stable PDDA-KPVS complex. (iii) Upon addition of KCl, the complex undergoes little change in Rh (62-67 nm) at a salt concentration (Cs)<or=0.02 M, aggregates with each other at Cs from 0.03 to 0.2 M, and becomes water-soluble at Cs>0.2 M. (iv) The Rh (78 nm) of the soluble complex at Cs from 0.3 to 0.5 M is larger than that at Cs<0.02 M, suggesting dissociation of the KPVS ions. (v) Complexation between KPVS and PDDA as mentioned in (ii) is facilitated in the presence of 0.01 M KCl.