This paper describes the synthesis of copper nanoparticles (NPs) inside sulfonated polyetherether ketone (SPEEK) membranes, which serve both as reaction media for intermatrix synthesis (IMS) of Cu-NPs and as stabilizing matrices, preventing NPs aggregation and uncontrollable growth. The IMS technique involves the loading of the functional groups of the polymer with metal ions followed by their reduction inside the polymeric matrix. It has been shown that the repetitive metal loading-reduction cycle, in the case of copper, is accompanied by comproportionation reaction between Cu0 and Cu2+, leading to the formation of Cu+ ions. This, in turn, allows for doubling the NPs content in the SPEEK matrix along with the tuning of Cu-NPs size. © 2010 American Chemical Society.