The pyrolytic conversion of an organosilsesquioxane (Owens-Illinois resin GR650) to SiO2 is characterized by ir spectroscopy, thermogravimetry and evolved gas analysis (line-of-sight mass spectroscopy). Scanning calorimetry, ramping at 10°C/min, on the as-received (room temperature annealed) resin indicates a glass transition temperature of 67°C which decreases to 58°C for an unrelaxed sample. The ir spectra have bands which can be assigned to Si-CH3 and Si-O-Si modes. For 30 minute isothermal anneals at temperatures above 420°C there is a continuous decrease in the bands associated with the Si-CH3 groups such that after 30 minutes at 650°C the ir spectrum has evolved to that for SiO2. Evolved gas analysis indicates that there are four major components evolving. Over the temperature range (ramping at 10°C/min) ∼180 to ∼500°C we observe C2H5OH and H2O, both of which are condensation reaction products from the curing reaction. Methane is a major evolving species over the temperature range ∼500 to ∼800°C and the thermal spectrum is double peaked which we attribute to CH3+ bound to the inside and outside of the polymer cage structures. The final major component detected was H2, over the temperature range ∼600 to ∼1100°C, which was attributed to pyrolysis of the organic components, both trapped and evolving. The features of the weight loss curve can be accounted for by the measured evolving species spectra.