Silane-based sol-gel coatings are commonly used as adhesion promoters. It is desirable to enhance the mechanical properties of these materials by reinforcing them with nanoparticulates, particularly oxide particles. Aggregation of the reinforcing particles is often associated with poor mechanical properties of the material. In this study, we evaluate the effect of the degree of aggregation of reinforcing oxide particles on the mechanical properties of glycidoxypropyltrimethyloxysilane (GPS) sol-gel coatings. The degree of aggregation is controlled by adjusting the surface electronic properties of the particles using tetrasodium pyrophosphate (TSPP), which specifically adsorbs to the particle surfaces. It is found that at small degree of aggregation (n_aggregate < 50 primary particles), there is only a negligible effect on the gel mechanical properties. However, large, loose aggregates caused by a more rapid aggregation leads to incoherent coatings with numerous cracks. This is a result of the participation of the oxide particles in the GPS gel network. Therefore, the structure of the gel formed is regulated by the structure of the oxide aggregates present. Large, loose aggregates lead to a formation of a weak, low-density gel that is susceptible to cracking during the drying process and poor mechanical properties.