Dynamic secondary ion mass spectrometry (SIMS), nuclear reaction analysis, and neutron reflectometry were used to profile polyimide-polyimide interfaces. For interfaces between two layers of poly(4,4’-oxydiphenylene-pyromellitimide) (PMDA-ODA) polyimide it was determined that the interfacial fracture energy G(c) and the interfacial width depended primarily upon the imide fraction, f, of the base layer. For f <0.9, there was a sharp interface between the deuterium-labeled poly(amic acid ethyl ester) (dPAE) and the base layer with a long low volume fraction tail of dPAE penetrating into the base layer. The volume fraction of the penetrant was limited by the imide fraction of the base layer and approached zero for f > 0.9. The PMDA-ODA/PMDA-ODA interface formed with a fully imidized base layer was similar to 30 Angstrom in width, whereas that formed when f = 0.9 was similar to 80 Angstrom in width. The interface formed during the spin-casting process did not broaden significantly after annealing at 400 degrees C. We also investigated the interface between a more flexible thermoplastic polyimide poly(4,4’-oxydiphenylene-oxydiphthalimide) (ODPA-ODA) and a fully imidized PMDA-ODA layer. The shape of the ODPA-ODA/PMDA-ODA interface was found to depend upon annealing temperature. Unannealed samples and samples annealed at 350 degrees C could be fit with an error function, and the interfacial width was similar to 50 Angstrom. Samples annealed at 400 and 450 degrees C were best fit by a fairly sharp interfacial profile with ODPA-ODA tails proceeding into the PMDA-ODA layer. The volume fraction of the ODPA-ODA in the PMDA-ODA depended upon the annealing temperature T-ann. The G(c) of the ODPA-ODA/PMDA-ODA interface appears limited by the solubility of ODPA-ODA in PMDA-ODA.