The interfacial area between the matrix and the filler is a key parameter which shapes the performance of polymer-based composites and nanocomposites, even though it is difficult to quantify. A very easy SAXS method, based on the Porod equation, is proposed for measuring the specific surface area of nanofillers embedded in a polymer matrix. In order to assess its reliability, this approach was applied to natural rubber-or styrene butadiene-based samples containing different types of montmorillonite clay. A wide range of specific surfaces was detected. SAXS data were compared to complementary X-ray diffraction and TEM information, obtaining a good agreement. Interpretation of the tensile properties by theoretical models and comparison with the literature corroborated the validity of the specific surface area measurement. The possibility to quantify this feature of composites allows the rational design of such materials to be improved.