The nanofiller, 1,3,5,7,9,11,14-hepta-2,2,4-trimethylpentyl-tricyclo[7.3.3.1(5,1 1)] heptasiloxane-endo-3,7,14-triol, T-POSS, was recently demonstrated to increase modulus retention in poly(ethylene terephthalate) fibers. In this paper, changes in the structure of T-POSS were studied as a function of time, temperature, and air vs. nitrogen atmosphere. These structural changes were studied using TGA, FTIR, MALDI-TOF, and NMR spectroscopy. T-POSS was observed to undergo an initial weight loss of approximately 8 wt% at temperatures below 250 degrees C under either atmosphere, which is attributed to loss of volatile contaminates in the starting silsesquioxane. Heating T-POSS to the PET processing temperature of 280 degrees C under nitrogen atmosphere results in production of a resinous organosilicate material which retains its isooctyl groups, and which contains, at least in part, structures with molecular weights greater than the starting silsesquioxane. Heating this resinous material to temperatures well above 280 degrees C then leads to loss of volatile products with residual char products present in less than 10 wt%. Heating T-POSS to temperatures of 280-1000 degrees C under air causes loss of organic species with 30-40 wt% residual inorganic silicate. While this study demonstrates that when T-POSS is exposed to a temperatures equivalent to those encountered during melt processing with polymers, such as PET, the exposure times during such processing is relatively low, generation of large yields of POSS decomposition products during melt processing is not consistent with the observed degradation kinetics.