The interfacial micromechanics of Twaron 2200 aramid fibers in an engineering thermoplastic elastomer (Pebax 7033, polyether amide block co-polymer) has been investigated by determining the distribution of interfacial shear stress along fibers in single-fiber model composites using Raman spectroscopy. The effects of various fiber surface treatments on the interfacial shear stress and fragmentation of the aramid fibers are discussed. The fiber average stress increased linearly with applied matrix stress up to first fracture. Each composite was subjected to incremental tensile loading up to full fragmentation, while the stress in the fiber was monitored at each level of the applied stress. It was shown that the experimental approach allowed us to discriminate between the strengths of the interfaces in the different surface-treated aramid fiber Pebax matrix systems, but also to detect different phenomena (interfacial debonding, matrix yielding and fiber fracture) related intimately to the nature of stress transfer in composite materials. The efficacy of the surface treatments was clear by comparing the maximum interfacial shear stress with the fragment lengths of the modified aramid fibers. The fiber breaks observed using Raman spectroscopy were not clean breaks as observed with carbon or glass fibers, but manifested themselves as apparent breaks by fiber skin failure. The regions of fiber fracture were also investigated using optical microscopy.