There is considerable interest in using carbon nanotubes (NTs) to create multifunctional polymer composite materials with outstanding mechanical, electrical, and thermal properties. A hurdle in modeling the behavior of these systems is the non-bulk interphase region in these systems that forms due to nanoscale interactions between the embedded NTs and adjacent polymer chains. This interphase region comprises a substantial portion of the volume fraction of the composite due to the tremendous NT surface area per unit volume available for interaction with local polymer chains and results in significant changes in the viscoelastic properties of the nanotube-polymer composite compared to those of the bulk polymer. However, the mechanical properties of this interphase region are unknown and very difficult to measure directly from experimental testing due to the size scale of this interphase region. Thus a three-phase (fiber/nanotube - annular interphase - matrix) Mori-Tanaka micromechanical model has been developed such that the properties of this interphase region can be inferred from macroscale viscoelastic data obtained using dynamic mechanical analysis. Such analysis will be particularly useful as a means to assess changes in the mobility and mechanical behavior of this interphase region and as a means to characterize the impact of chemical functionalization on interphase formation in these systems.