In this study the positive and negative static secondary ion mass spectrometry (SSIMS) spectra of pyromellitic dianhydride−4,4′-oxydianiline (PMDA-ODA) polyimide before and after imidization were analyzed. Isotopically labeled materials were used to deduce the structure and source of secondary ions produced by this material. A signature for imidization of the surface layer is given by disappearance of fragment ions attributed to ODA and PMDA end groups from the positive SSIMS spectrum. In addition to confirming our speculations as to the source of secondary ions, we found that addition of free ODA dramatically increases the amount of ODA end groups after curing. This, in concert with observations that include invariance of the 14N to 15N ratio with film depth, appearance of a new high-temperature weight loss region in the thermogravimetric analysis (TGA) of films containing excess ODA, and overall molecular weight reduction for cured films, leads us to propose a cascading mechanism of excess amine incorporation and equilibration throughout the film. © 1991, American Chemical Society. All rights reserved.