Mesostructured forms of silica (denoted MSU-J) and aminopropyl-functionalized silica (denoted AP-MSU-J) with wormhole framework structures are effective reinforcing agents for a rubbery epoxy polymer. At loadings of 2.0–10 wt %, MSU-J silica with an average framework pore size of 14 nm (65 °C assembly temperature) provides superior reinforcement properties in comparison to MSU-J silica with a smaller average framework pore size of 5.3 nm (25 °C assembly temperature), even though the surface area of the larger pore mesostructure (670 m2 g−1) is substantially lower than the smaller pore mesostructure (964 m2 g−1). The introduction of 5.0 and 10 mol % aminopropyl groups in the wormhole framework walls decreases the textural properties in comparison to the pure silica analogs. AP-MSU-J organosilicas increase the tensile strength as well as the strain-at-break of the rubbery epoxy mesocomposites in comparison to MSU-J silica as a reinforcing agent. The improved toughness provided by the aminopropyl functionalized mesostructures is attributable in part to covalent bond formation between the mesostructured silica walls and the cured epoxy matrix and to a more ductile mesostructure framework in comparison to a pure silica framework. An organosilica derivative containing 20 mol % aminopropyl groups, but lacking a mesostructured framework, provides little or no improvement in polymer tensile properties, demonstrating that an ordered porous network is essential for polymer reinforcement. In general, the reinforcement benefits provided by mesostructures with larger framework pores are superior to those provided by smaller pore derivatives, most likely because of more efficient polymer impregnation of the particle mesopores. The presence of a mesostructured form of the organosilica is essential for improving the mechanical properties of the epoxy polymer.