We examined the vapor deposition of polyimide produced by co-evaporation of pyromellitic dianhydride and oxydianiline. Using internal reflectance spectroscopy and prism coupling techniques, we monitored the effects of varying relative monomer fluxes on cured film stoichiometry. We observed the effects of monomer interdiffusion during the ex-situ curing leading to segregation of partially reacted species at the film/substrate interface. Such interface segregation may be influential in determining adhesion of polyimide to metal layers for microelectronics applications.