The composition and microstructure of the interface in the copper-polyimide (PI) system formed in the process of electroless copper plating from solutions onto a PI film surface activated by tin and palladium compounds have been studied. It was found that the transition region in the near surface of the PI film layer was 20-30 nm in thickness and consisted of PI globules, copper and palladium particles, oxides of these metals, and Sn(II) and Sn(IV) oxo- and hydroxo-compounds. This region was formed as a result of metal ion adsorption and diffusion into the swelled PI layer, resulting in PI globule junctions which serve as capillaries. These processes were accompanied by the formation of C-O-Me and Me-Ni bonds due to ion exchange or donor-acceptor interaction of metal ions with carboxyl or amido groups generated during PI etching in alkali solutions. The formation of fine tin hydroxo-compounds and copper and palladium particles of nanometre sizes which are present on the whole substrate surface accessible to solutions of concentration up to 1000 mu m(-2) or higher plays an important role in providing strong (700 N/m) copper-to-polyimide adhesion.