Multiwalled carbon nanotubes (MWNTs) were oxidated and functionalized in order to determine the structural and chemical changes on their atomic bonding. MWNTs were oxidized in an aqueous solution of HNO 3 (70%) and H 2 SO 4 (95%) at 25 °C. Pristine and oxidized carbon nanotubes (ox-MWNTs) were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and thermogravimetrical analysis (TGA). MWNTs and ox-MWNTs were silane-functionalized and incorporated into a polymethyl-methacrylate (PMMA) matrix. The dispersion properties and interface interactions were studied for the composite materials. MWCNTs oxidation was performed and supported by Raman spectroscopy and FTIR results of disorder and defects in the carbon lattice; as well as change in the amount and type of chemical groups attached to the CNTs walls. Roughness areas visible in SEM images indicate Van der Waals interactions between CNTs and functionalization reagents. Control of the oxidation state can effectively direct molecular functionalization of a CNT sidewall. Composites showed good dispersion of filler; with the best one for oxidized-silane functionalized CNTs.