We investigate the mechanical property predictions for a three-phase viscoelastic (VE) composite by the use of two micromechanical models: the original Mori–Tanaka (MT) method and an extension of the Mori–Tanaka solution developed by Benveniste to treat fibers with interphase regions. These micro-mechanical solutions were compared to a suitable finite-element analysis, which provided the benchmark numerical results for a periodic array of inclusions. Several case studies compare the composite moduli predicted by each of these methods, highlighting the role of the interphase. We show that the MT method, in general, provides the better micromechanical approximation of the viscoelastic behavior of the composite; however, the micromechanical methods only provide an order-of-magnitude approximation for the effective moduli. Finally, these methods were used to study the physical aging of a viscoelastic composite. The results imply that the existence of an interphase region, with viscoelastic moduli different from those of the bulk matrix, is not responsible for the difference in the shift rates, μ22 and μ66, describing the transverse Young's axial shear moduli, found experimentally.