Flocculation of colloidal silica with aluminum fractal polymers was investigated by Al-27 magic angle spinning and Si-29 cross polarization magic angle spinning NMR as a function of aluminum concentration and pH. Aluminum flocculant species were prepared by dilution of a commercially available flocculant, WAC HE, the hydrolysis of which yields Al-13 polymers. The results showed that destabilization with hydrolyzed aluminum has many features in common with flocculation by addition of conventional organic polymers. Interaction of aluminum polymers with silica leads to the formation of four coordinated aluminum retained at the silica surface as negatively charged aluminosilicate sites. These sites, similar to those found in clay minerals and zeolitic materials, represent potential anchors to aluminum polycations. Hence, aggregation of silica particles proceeds with either charge neutralization or bridging. Tetrahedral aluminum in contact with silica may be assimilated to polymer segments bound to the surface. Study of the effect of aluminum concentration and pH suggests that aluminum partition within silica flocs may be ascribed to a competition between structural rearrangement of individual aluminum polymers, which tend to adopt a flat conformation on the silica surface, and excluded area effects originating from neighboring flocculant species.