Polyhedral Oligomeric Silsesquioxanes (POSSs) of general formula R7R'(SiO1.5)(8), where R= isobutyl, cyclopentyl or phenyl and R' = -(CH2)(5)-CH3-(CH2)(7)-CH3 or (CH2)(9)-CH3-, were prepared by corner capping reaction of trisilanols with suitable allcyltriethoxysilane. The compounds obtained were characterized by elemental analysis and H-1 NMR spectroscopy, and the results were in very good agreement with those of expected products. The prepared POSSs were degraded in dynamic heating conditions from r.t. to 700 degrees C, in both inert and oxidative atmospheres, and the temperatures at 5% weight loss (T-5%) were determined and compared with each other, in order to investigate how the various groups affect the resistance to thermal degradation. The results did not evidence any substantial difference between the two investigated atmospheres, but indicated clearly an increasing resistance to the thermal degradation according to the following order: isobutyl POSS5 < cyclopentyl POSSs < phenyl POSSs. Also, T5% values were linearly increasing as a function of the number of methylene groups in R' alkyl chain. The results were discussed and suggested a group contribution to thermal stability. (C) 2015 Elsevier B.V. All rights reserved.