The interaction between polyphosphates and polyamines was investigated by 31P-NMR spectroscopy and by amine oxidase activity measurements. An apparent competition between negatively charged polyphosphates (ATP, ADP, AMP, tripolyphosphate and pyrophosphate) and positively charged polyamine, for the active site of bovine serum and soybean seedling amine oxidases, was observed by activity measurements. This behavior was explained by formation of polyamine-polyphosphate complexes and the stability constants of these complexes were calculated by 31P NMR. However, at a given concentration of polyphosphate, the amine oxidase activity was found higher than that expected on the basis of the free amine concentration calculated according to the NMR stability constant. This fact, and the different extent of inhibition of the spermidine oxidase activity of soybean seedling and of bovine serum amine oxidases observed in the presence of a given polyphosphate, suggest that amine oxidases may be active also on the polyamine-polyphosphate complexes. This hypothesis was supported by the strong dependence of the kcat/Km of bovine serum amine oxidase on ionic strength, indicating an electrostatic interaction between the charged amine and the active site, while no effect of ionic strength on kcat/Km was observed in the presence of ATP. A kinetic model of this behavior was found to fit the experimental data.