Fourier transform infrared spectroscopy (FTIR) was used as a novel characterization method to determine the properties of the interface that developed when cobalt oxide nanoparticles were self-assembled in a poly(methyl methacrylate) (PMMA) matrix. The method employed the distinct changes that were observed in the infrared spectra of the polymer upon adsorption onto the cobalt oxide nanoparticles, allowing a quantitative determination of the average number of contact points that the average polymer chain formed with the surface of a cobalt oxide nanoparticle of average size. The results obtained with this method compared favorably to those obtained by the coupling of transmission electron microscopy (TEM) experiments with thermogravimetric analysis (TGA). On the basis of both methods, we concluded that the interfacial region created between the cobalt oxide nanoparticles and PMMA is extremely sensitive to the chain length, i.e., the number of anchor points and the density of the polymer layer increase with chain molecular weight. At molecular weights of similar to 250 000, the density of the polymer layer saturates at a value that correspond to that of very thin PMMA films.