Admixture of aluminum nitrate, sodium polyphosphate, and ammonium hydroxide solutions yields stable dispersions of hydrated aluminum polyphosphate particles within a broad reagent concentration range. These particles are formed by liquid-liquid phase separation, for which a phase diagram was calculated using suitable models for concentrated electrolyte solutions. Particle effective diameters range from a few nanometers to many hundreds and are fractionated by centrifugation. Particle electrophoretic mobility is very low and the hydration degree is high (similar to 80% v/v). Dry nanoparticles (1- to 15-nm diameter as observed by TEM) as well as particle aggregates are obtained by lyophilization. Element (P, Al, and Na) mapping by ESI-TEM shows that particle aggregates have a core-and-shell morphology, with a higher content of P in the aggregate core and a higher Na content at the outer shell.