Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy were used to monitor the chemical composition of both ladder and cage hydrogen silsesquioxane (HSQ) as a function of bake and cure temperature in the range 150–425°C. Three distinct chemical compositions evolve as the temperature of the HSQ film is increased. Kinetic and spectral evidence is provided which supports the redistribution of Si‒O and Si‒H bonds as the reaction mechanism responsible for the chemical changes induced by heating. Dielectric constants are reported for both the ladder and cage HSQ and are related to the composition changes that accompany thermal treatment. A comparison is made of the thermal stability of the cage and ladder HSQ as it relates to the net dielectric constant at the conclusion of thin film interconnect processing. The ladder HSQ is found to be thermally more stable in the temperature range studied. The dielectric constant of the cage HSQ is considerably degraded by furnace cure processes.