This work aimed to obtain information on the water dispersibility of a 1:1 stoichiometric polyelectrolyte nanogel complex (SPENC). We synthesized a cationic polyelectrolyte nanogel (CPENG) composed of a cross-linked copolymer of 1-vinylimidazole and N-isopropylacrylamide. SPENC was then prepared at 25 degrees C from the mixing of equimolar amounts (based on fixed charges) of CPENG and potassium poly(vinyl alcohol) sulfate, which were dissolved in an aqueous solution without adding salt and at pH 3.0. We carefully observed at 25 degrees C the reduction of the imidazole-based cationic charge in the CPENG component of SPENC as a function of pH. Dynamic and static light scattering techniques were employed in combination with electrophoretic light scattering experiments. The amount of cationic charge in the SPENC was estimated from the potentiometric titration data of CPENG. It was found that, during the charge reduction process, the complex underwent aggregation, followed by a phase separation. The aggregation started at about 25% of the charge reduction (i.e., at pH approximate to 4.9), and the phase separation took place when almost half of the charge was eliminated (at pH approximate to 5.5). However, the phase-separated complexes became water-soluble again when about 90% of the charge was eliminated (pH approximate to 6.6). By colloid titration, the dissociated free polyanions were not detected in the aqueous SPENC solution before the phase separation but were detected in the complex-redispersed solution. When the pH (9.0) of the redispersion was slowly decreased to the original level (pH 3.0) by the gradual addition of HCl so as to cause again the phase separation, an intraparticle complex was reformed, the physical quantities of which were close to those of the initial SPENC. These findings clearly indicate that the whole and a part (segment) of the complexed polyanions undergoes dissociation-association reactions on the surface of a SPENC particle, depending on the ionization state of the cationic gel component. As a result, these reactions seem to be a key factor for the water dispersibility of the SPENC.