Polyelectrolyte complexes (PECs) with pH-tunable solubility were formed from a random copolymer of diallyldimethylammonium (DADMA) and acrylic acid (AA) having a net positive charge as polycation and poly(styrenesulfonate) as polyanion. Quasi-soluble PEC nanoparticles could be produced from solutions of exceptionally high polymer concentration by manipulating pH and ionic strength. The aggregation of the PEC nanoparticles, reversible using either changes in salt concentration or pH, exhibited well-defined hysteresis. Dynamic and static light scattering as well as transmission electron microscopy showed well-defined particles produced in the quasi-soluble limit. Viscosity and titration experiments supported a core-shell model for the dispersed nanoparticles and revealed preferential ion pairing of DADMA with sulfonic acid groups over AA units.