Sol?gel materials are often reinforced through the addition of particulate fillers to improve their mechanical properties, such as toughness or stiffness. Related studies of filled polymeric materials suggest that the effectiveness of added reinforcements may depend sensitively on the state of particulate aggregation in the composites. In some systems and situations, aggregates are found to enhance properties, while in others they are deleterious. The objective of the present study was to determine the effect of state of aggregation of alumina nanoparticle fillers on the mechanical properties of a GPS sol?gel hybrid composite. Aggregates of different sizes were produced at loadings of 3.6, 7.5, and 16.8 volume percent in the GPS sol?gel monoliths by inducing aggregation through the addition of varying amounts of tetrasodium pyrophospate (TSPP). The small amounts of TSPP required to induce varying states of aggregation did not affect the properties of the gel in the absence of the particles. Unaggregated particles or small aggregates were found to impart significant improvements in the mechanical properties of the gel, but at a particular critical aggregate size and beyond, the cured material cracked and crumbled. The critical size was found to correspond roughly to an effective aggregate volume fraction of 0.64 in the final composite, suggestive of a randomly close-packed aggregate structure.