The intrinsic and ambient-induced thermal decomposition mechanisms in low dielectric-constant (k) fluorocarbon films have been studied by interferometry for thermal stability. The fluorocarbon films were deposited by pulsed plasma chemical vapor deposition from tetrafluoroethylene. Pulsing the plasma allows films with various degrees of cross-linking and free radicals to be produced. In situ annealing in fluorine and inert environments give small improvements in postanneal thermal stability. However, substantial network rearrangement and film loss must occur in order to improve the thermal stability of the resulting annealed material. The effect of various ambient gases on decomposition during ex situ heating was also studied. Oxygen incorporates into highly crosslinked films, reducing thermal stability. However, oxygen is less detrimental to the thermal stability of films having mole linear chain structure. Fluorine, while able to provide a small degree of free radical capping, predominately etches the films at high temperatures. Hydrogen slightly reduces stability in linear chain films by forming low molecular weight fragments. In highly crosslinked films, hydrogen and nitrogen anneals give similar outcomes. In both in situ and ex situ cases, the degree of cross-linking in the film plays a role in determining the mechanism of decomposition, with these moderately cross-linked films falling in a minimum of stability.