The interphase between the inorganic filler and the polymer matrix is considered as one of the most important parameters for inorganic/polymer nanocomposites. In nanocomposites based on semicrystalline polymers we have to take into account two, often immobilized, interphases. One is existing between the polymer crystals and the amorphous part of the polymer and another one between the inorganic filler and the polymer matrix. The fraction of immobilized interphases, the rigid amorphous fraction (RAF), is available from specific heat capacity at the glass transition region of the nanocomposites. Assuming a certain constant amount of RAF around the individual lamellar crystals, which is available from the pure semicrystalline polymer, we determined the RAF related to the inorganic nanofiller. For the polyamide 6/layered silicate nanocomposites studied the RAF due to the filler reached more than 25% of the polymer fraction. The existence of a RAF around the nanofiller finally explains the observed reduced crystallinity in the polymer and the retarded nonisothermal crystallization of the polyamide 6 in the nanocomposites.