Industrial and model polymer nanocomposites are often formulated with coating agents (CAs) to improve polymer-nanoparticle (NP) compatibility. Here, the localization of silane CAs in the styrene-butadiene nanocomposite is investigated through the segmental dynamics of the polymer matrix by broadband dielectric spectroscopy (BDS), allowing the detection of silanes in the matrix through their plasticization effect. This acceleration of dynamics was followed via the shift of tau(max) of the alpha-relaxation induced by the presence of CAs of different molecular weights and quantities, for different amounts of incorporated colloidal silica NPs (R approximate to 12.5 nm, polydispersity 12%). Any noteworthy contribution of interfacial polymer layers to tau(max) has been excluded by reference measurements with bare NPs. Our approach allowed quantifying the partition between the matrix and the NP interfaces and was confirmed independently by calorimetry. As a control parameter, the silane-grafting reaction could be activated or not, which was confirmed by the absence (resp. presence) of partitioning with the matrix. Our main result is that in the first steps of material formulation, before any grafting reaction, CAs both cover the silica surface by adsorption and mix with the polymer matrix-in particular if the latter has chemical compatibility via its functional groups. Silane adsorption was found to be comparable to the grafted amount (1.1 nm(-2)) and does not increase further, confirming that the plateau of the adsorption isotherm is reached in industrial formulations. These results are hoped to contribute to a better understanding of the surface reactions taking place during complex formulation processes of nanocomposites, namely, the exact amounts at stake, for example, in industrial mixers. Final material properties are affected both through NP-matrix compatibility and plasticization of the latter by unreacted molecules.