Nylon 6/clay nanocomposites (N6CN) with different cooling histories were investigated by differential scanning calorimetry (DSC) and variable-temperature X-ray diffraction (XRD). Above the melting temperature, new endothermic peaks appeared in the DSC trace for N6CN. All the neat nylon 6 samples presented amorphous XRD patterns when heated up to the melting range. However, for N6CN samples, undefined crystalline structures remained in the substantially molten polymer matrix up to 300 degrees C.