Orientation of cellulose nanowhiskers (CNWs) within various matrices is an effective approach that affords the exploitation of the high stiffness of CNWs as a reinforcement phase in nanocomposites. One potential engineering method to achieve orientation is to incorporate magnetic properties into the CNWs by tethering magnetic nanoparticles onto the surface of these nanoparticles, followed by their alignment in an externally-applied magnetic field. In this work we report the successful in-situ tethering of iron oxide nanoparticles (IONPs) onto CNWs by the thermal decomposition of Fe(CO)5 in a H2O/DMF suspension. Following this procedure, IONPs consisting of mixtures of Fe3O4 and Fe2O3 with an average diameter of 20 nm were attached to the CNWs. The type of iron oxide species that was generated was determined by selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS) and the particle size was evaluated by transmission electron microscopy (TEM). Raman spectroscopy was used to characterize the presence and the nature of the molecular interaction between the IONPs and the CNWs.