Three routes to the preparation of nanodispersed metal or metal oxides particles in polymer matrices have been studied. Thermal treatment in air of W-, Mo-and Cr-carbonyl complexes with polyacrylonitrile (PAN) results in the formation of nanometer-size metal oxide particles in a polycyclic polymer. Co-containing polymers were prepared by mixing Co 2 (CO) 8 with a PAN copolymer or an aromatic polyamide in dimethylformamide (DMF). Co 2 (CO) 8 interacts with DMF giving the complex [Co(DMF) 6 ] 2+ [Co(CO) 4 ] -2 . Subsequent thermolysis converts this complex to nanodispersed Co particles. By ferromagnetic resonance and small-angle x-ray scattering, it was found that the average Co particle size depends on the type of polymeric matrix, the thermolysis conditions and the Co loading and varies from 1 to 10 nm.Colloidal metal particles and metal clusters are intriguing because their behavior differs from both bulk metal and isolated metal atoms. The huge specific surface and confinement of charge carriers suggests potential applications as catalysts, as ferrofluids, and as materials for third- harmonic generation (1,2), The principal difficulty in preparing such metal colloids is developing a method to control the particle growth. This problem can be partly solved by carrying out the nucleation and growth process either in solid polymer matrices (3) or in cages (or pores, for example, zeolites) (4) or in organized media such as microemulsions, vesicles or micelles (5). Reactions in such microreactors work with the concept that growth processes are limited in nanostructured matrices by the size of the structures themselves. Here, we discuss another way to control the nucleation and growth process in polymeric matrices which might be especially appealing when the methods listed above cannot be applied, that is, when the polymeric matrix does not form vesicles or micelles. For this case, carrying out the metal colloid formation in solid polymeric matrices can provide size control by changing the type of polymeric matrix, complexation power