Raman spectroscopy was used to map the stress transfer at the interface between high and low modulus carbon fibres in model composites when undergoing fragmentation. Both fibre surfaces were coated with two types of single wall carbon nanotubes (HiPCO and carboxylated nanotubes) in order to enhance the interfacial shear strength with an epoxy resin. For the low modulus carbon fibre this coating also enabled stress mapping at the interface. In both cases single fibres embedded in a dumbbell shaped model composite were deformed to cause fragmentation. When no further fragmentation took place the critical fibre length was calculated and converted to interfacial shear stress using classical Kelly-Tyson theory. These values were compared to data obtained using a Raman spectroscopic approach where the rate of change of stress with respect to distance along the fibre was measured directly. These data were then shown to fit a shear lag model. Two forms of single-wall carbon nanotubes were compared; namely unmodified and COOH modified. It was shown that only the COOH modified single wall carbon nanotubes increase the maximum interfacial shear stress significantly. Evidence of matrix yielding at the fibre ends is also presented and the possibility of the enhancement of the shear yield stress of the resin by the presence of the nanotubes is also discussed. © 2014 Elsevier Ltd.