Single-walled carbon nanotubes was purified and modified with dichlorocarbene, HNO3 or octadecylamine and incorporated in polymeric matrices of polycarbonate and epoxy. Exposing the composite material to a tensile strain and measuring the shift of the G'-band revealed that higher loads could be transferred to the HNO3-modified carbon nanotubes. Exfoliation was regarded as the main reason for the increased interfacial strength resulting in larger specific surface area and higher hydrostatic compressive stresses on the nanotube bundles.