Thermal decomposition of new silsesquioxane materials [(Me3Si)(3)CSiMe2CH2CH2SiO3/2](n) (PTSiSS), substituted with sterically hindered carbosilane groups, was studied in an inert atmosphere (N-2) and air. It was found that a specific degradation of carbosilane moieties occurs at high temperatures. A ceramic residue was obtained > 900 K both in nitrogen (SixOyCz) and air (SixOy). Thermal rearrangements in the structure of polyhedral and ladder-like PTSiSS were studied, and related to those observed for a polymethylsilsesquioxane resin (PMSS) of regular ladder-like architecture. Thermally induced transformations were evaluated by TGA(DTA)-MS, FTIR, NMR, XRD and SEM. (C) 2009 Elsevier B.V. All rights reserved.