Six different silica-polyethylene glycol (SiO2/PEG) organic-inorganic hybrid nanocomposites with different amounts of PEG 400 (6, 12, 24, 50, 60 and 70 wt.%) were synthesized by the sol-gel technique. Their thermal behavior was studied by thermogravimetry (TG) and differential thermal analysis (DTA) under a flowing argon atmosphere in a wide temperature range and their behavior was compared with those of the related materials with 60 and 70 wt.% of PEG, whose results were reported in a previously study. To identify all physical and chemical processes occurring in these promising materials, the gases evolved during TG experiments were analyzed at characteristic constant temperatures by FTIR. These measurements revealed that all the materials undergo a two-step dehydration (loosing water physically bound at temperature lower than 100 degrees C) with overlapping of both TG and DTA curves, thus suggesting an identical behavior up to 200 degrees C regardless the different content of PEG. A different behavior is observed at higher temperature, where three different exothermic effects were observed in the range 200-450 degrees C accompanied by mass losses ranging from 6 to 37% for the SiO2/PEG hybrid materials. POLYM. ENG. SCI., 57:606-612, 2017. (c) 2017 Society of Plastics Engineers