The structure of the interphase, a region between nanoparticle fillers and the bulk polymer matrix in a particle reinforced composite, was investigated using two different approaches. The polymer nanocomposite systems consists of alumina ( Al2O3) and magnetite (Fe3O4) nanoparticles embedded in poly( methyl methacrylate) ( PMMA) and polystyrene ( PS) matrices. The first approach utilized data from thermal gravimetric analysis ( TGA) and transmission electron microscopy ( TEM) to predict the structure and density of the interphase for four nanocomposite systems. In the second approach, the nature of bonding between the polymer and the nanoparticle surfaces was analyzed using Fourier transform infrared spectroscopy ( FTIR) to calculate the density of the interphase for two PMMA- based nanocomposite systems. Mechanical properties of these composites were correlated with the structure of the interface, and results from the two approaches were compared with previous studies. Moreover, by comparing results from the two characterization approaches, a new method for characterizing the degree of nanoparticle flocculation in a composite is also provided. The results indicate that Al2O3 nanoparticles are more reactive with the polymer matrix than Fe3O4 nanoparticles, but neither have strong interactions with the matrix, a fact that leads to low- density interphase and consequently results in more compliant composites. Tensile testing, dynamic mechanical analysis ( DMA), and nanoindentation tests confirmed that these nanocomposite systems do not have the same mechanical properties as their respective pure polymer systems.