The integrity of an interface between a polyimide and an epoxy is important for a number of microelectronic applications. Here we investigate a surface-modification procedure for the polyimide designed to strengthen its interface with a silica-filled epoxy. The polyimide is chemically modified so that a thin surface layer is converted back to its polyamic acid (PAA) precursor. The modified polyimide is then coated with a solution of the epoxy before dispensing the normal silica-filled epoxy. Interpenetration of the epoxy into the solvent-swollen PAA layer enhances the entanglements between the polymer chains and/or increases the number of primary covalent bonds across the final interface. Increasing the thickness of the PAA layer [measured by Rutherford back scattering spectrometry (RBS)] leads to an increased interpenetration w [measured by secondary ion mass spectrometry (SIMS)]. The fracture energy (G(c)*) of the unmodified polyimide/silica-filled epoxy interface ( w = 12 nm) is very low (similar to 25 J/m(2)), but G(c)* increases with w until w = 44 nm (G(c)* approximate to 100 J/m(2)). Increasing w further leads to an interface that is tougher than the epoxy. The interface can also fail subcritically because of stress-assisted attack of water in the environment (SAWA) on the primary covalent bonds across the interface. We find that increasing w from 12 nm to 38 nm improves the resistance of the interface to SAWA, with the threshold energy release rate (G(th)*) for SAWA increasing by more than sixfold and the steady-state water transport controlled subcritical crack growth velocity ( v *) well above G(th)*, decreasing by nearly two orders of magnitude. SIMS is used to probe the fracture surfaces to determine the locus of the crack growth in both our fracture energy samples and our subcritical crack growth samples. In both cases, the interface with an unmodified polyimide surface fails along the interface, and the modified interfaces (12 nm < w < 44 nm) typically fail within the newly created interpenetrating epoxy/polyimide interphase.