This paper describes kinetic and mechanistic studies of the homogeneous formation of highperformance polyimides. Controlled molecular weights and nonreactive phthalimide end groups were emphasized. NMR spectroscopy, nonaqueous titration, and solution viscosity measurements were the principal techniques utilized. The disappearance of the amic acid groups was followed to quantitative conversion via nonaqueous titration with tetramethylammonium hydroxide. The homogeneous solution imidization processes were well described by auto-acid-catalyzed second-order kinetics. The effects of heteroatom electron-donating and -withdrawing bridging groups (0 and SOz) in the diamines and dianhydrides on imidization rates were investigated, and a possible reaction mechanism for the solution imidization processes was proposed. Imidization of 4,4'-oxydianiline/4,4'-oxydiphthalic anhydride poly(amic acid) was investigated at three different temperatures (140,150, and 180 O C ) . Two-dimensional 'H-lH correlation spectroscopy and intrinsic viscosity measurements provided direct evidence for partial degradation of the poly(amic acid) backbone structures. Complete imidization, including rehealing of the broken chains, was achieved under proper reaction conditions, and the implication of these findings was considered to be very significant for the synthesis of thermally stable high-performance polyimides. For polyimide systems containing benzophenonetetracarboxylic dianhydride (BTDA), direct evidence for network formation involving imine branching and cross-linking was observed by high-field lH NMR spectroscopy and the formation of gels was discussed as a function of reaction conditions. One might anticipate that all ketone-containing polyimides could, to varying degrees, display this behavior.