All-cellulose nanocomposites are produced using dissolved microcrystalline cellulose (MCC) as the matrix and cellulose nanowhiskers (CNWs), produced by acid hydrolysis, as the reinforcement. These nanocomposites are then characterised using X-ray diffraction to determine their crystallinity, and Raman spectroscopy to discriminate the reinforcing phase (cellulose I) from the CNWs and the matrix phase (cellulose II) from the dissolved MCC. Mechanical testing of the composites shows that there is a significant systematic reinforcement of the matrix material with the addition of CNWs. Furthermore, Raman spectroscopy is used to show that distinct spectroscopic bands for each phase within the composite spectrum can be used to discriminate the effects of both reinforcement and matrix. It is shown that a Raman band located approximately at 1095cm−1 can be used to follow the micromechanical deformation of the CNWs and matrix, whereas another band located at 895cm−1 arises purely from the matrix. This spectroscopic fingerprint is used to gain insights into the complex interactions occurring in these potentially recyclable composite materials, and offers a way forward to optimising their properties.