Both low and high modulus carbon fibres are coated with carboxylated single wall carbon nanotubes (SWNTs). It is shown that it is then possible to follow, for the first time, the local deformation of low modulus carbon fibres and composite interfaces using Raman spectroscopy. By deforming coated single carbon fibre filaments in tension, and following the shift in the position of a band located at ~2660cm -1 (2D band) it is possible to calibrate the local stress state of a fibre embedded in an epoxy resin. To follow the interface between the fibres and the epoxy resin, a thin film model composite is used. Point-to-point variation of stress along a single fibre, both inside and outside the resin, is recorded and stress transfer models are used to determine the interfacial shear stress (ISS). Values of the ISS (~20MPa) are obtained for the thin film model composites for untreated high modulus fibres. A beneficial interfacial effect of the presence of SWNTs on the surface of the high modulus carbon fibre samples is demonstrated resulting in an increase in the maximum ISS ( > 30MPa) compared to uncoated samples. Similarly coated low modulus fibres exhibit a very high ISS ( > 50MPa). These increases are attributed to an enhanced contact between the resin and the fibres due to an increased surface area as a result of the nanotubes and additional bonding caused due to the presence of carboxylate groups. © 2014 Elsevier Ltd.