In this study, in situ modified phenol formaldehyde resins were prepared from clay (sepiolite) in the presence of base catalyst. Different clay contents (3 wt%, 5wt%, 8 wt%, 10 wt%, 15wt%, 20wt%, 30wt% and 50 wt%) were used to produce sepiolite modified resol nanocomposite resins (SEP-PFNCRs). SEP-PFNCRs were partially cured by heat and the effects of the curing process and the clay content in the resol resin were determined on the spectroscopic, thermal, and microscopic properties of the final products. The structures of the specimens were characterized by means of Fourier Transform Infrared (FTIR-ATR) spectroscopy. Thermal properties of the samples were determined with Thermogravimetric Analyzer (TGA). The dispersion of sepiolite in the resol nanocomposites were examined by X-ray Diffraction (XRD). The obtained samples were also characterized morphologically by Scanning Electron Microscope (SEM). (C) 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).