In this thesis, the thermal degradation of poly(ethylene oxide)/phenyl hepta isobutyl polyhedral oligomer silesquioxane (PEO / f, hib-POSS) nanocomposites was investigated, that is, the influence of the addition of 5% nanopowder on the thermal degradation of PEO by thermogravimetric method under dynamic conditions at four different heating rates. The effect of five monosubstituted phenyl hepta isobutyl oligomeric polyhedral silesquioxanes was investigated: phenyl hib-POSS (POSS 1), 4-methyl hib-POSS (POSS 2), 4- methoxy hib-POSS (POSS 3), 4-fluoro hib-POSS ( POSS 4) and 4-chloro hib-POSS (POSS 5). Poly(ethylene oxide) has been found to decompose in one decomposition step in the temperature range from 330 to 450 °C. PEO/f, hib-POSS nanocomposites are degraded through two degradation stages. The first degradation step takes place in the temperature range of 200 - 250 °C, with the sample losing only 2 - 6% of the initial mass, depending on the rate of heating. The second degradation stage of the nanocomposites takes place in the same temperature range as the degradation of pure PEO. TG analysis has determined that the addition of nanoparticles does not affect the thermal decomposition of poly(ethylene oxide) at temperatures important for processing and application, up to 200 °C. Above this temperature, the addition of f,hib-POSS nanoparticles accelerates the thermal degradation of poly(ethylene oxide). 4-methyl hib-POSS (POSS 2) showed the greatest influence, while 4-chloro hib-POSS (POSS 5) had the least influence on the thermal stability of the nanocomposites under study.