Calorimetry and high-performance refractometry are applied to study the network formation and accompanying morphological changes in reactive polydimethylsiloxane systems filled with silica nanoparticles. Both methods give insight into the structure formation during the polymerization of silica-filled silicone nanocomposites in an impressively complementary way. It will be shown that the specific heat flow as determined by calorimetry does not allow for estimating the chemical conversion whereas the refractive index does, which results from the different perspectives of both techniques on the relevant polymerization, adsorption, and desorption processes occurring in the nanocomposites. Finally, the interphase formation in the polymeric matrix in the vicinity of the nanoparticles and the network formation within the silicone rubber are demonstrated to be strongly correlated processes.