We presented a new framework for the mechanical characterization of polymer nanocomposite interphases using AFM nanomechanical mapping. Specifically, by experimental model system, we extended the deconvolution method of "substrate effect" from Collinson, et al. We developed a polymeric model composite including an artificial interphase with tunable thickness and mechanical properties. We mapped the mechanical properties of these systems across the thickness of the interphase with three different probe sizes using AFM Fast Force Mapping. From the measurement results, we constructed modulus-distance profiles for each combination of interphase thickness and probe size. Our results validate the FEA master curve predictions of Collinson, et al. that account for the substrate effect in absence of an interphase. We showed that the presence of an interphase of known thickness causes a systematic, thickness-dependent deviation of the modulus-distance profiles from the predictions of the master curve. Furthermore, we showed that this deviation of the experimental reduced modulus-distance profiles from the prediction of the master curve, i.e., (E/EFF-EMC), can be reliably fit with a sigmoidal curve if the ratio of interphase thickness to probe radius is sufficiently large, i.e., (λphase /aFF) > 3. This approach allowed us to accurately predict the interphase thickness in our model system solely from nanomechanical measurements using AFM probes with known radii. Finally, we showed that our approach is applicable for the determination of the nanomechanical properties of a broad range of soft material interphases by properly accounting for the "substrate effect."