The decomposition in vacuum of a cured polyimide resin (Skybond 700) was studied in detail using simultaneous mass spectral and differential thermal analysis. These data were complemented with kinetic studies of the polymer and selected model imides. The mechanisms involved in the decomposition of Skybond 700 at moderate temperatures were determined using the combined results. It was discovered that the major endotherm seen in the thermogram was due to three simultaneous reactions: 1) Rupture of the benzophenone carbonyl bond to yield CO, 2) rupture of the imide bond to yield CO, and 3) rupture of the imide bond to yield CO2. Reaction 3 is in conflict with previous studies where CO2 was reported to arise from impurities in the polymer. The imide linkage as a source of C02 was substantiated by studying the degradation of model imide compounds. The decomposition rate of each of the models, in the vapor phase, was found to be first order, as was the rate of formation of CO2. It was then postulated that CO3is formed in the decomposition of the imide linkage by an intromolecular rearrangement through an ionic isomeric intermediate. Reaction rates were determined for the models, and the activation energies for each of the competing decomposition reactions for N-phenylphthalimide, the model most closely related to the polymer imide linkage, were calculated.