Oxidatively stable, magnetic cobalt nanoparticles have been prepared by heating cobalt nanoparticles encapsulated in poly(styrene-b-4-vinylphenoxyphthalonitrile) block copolymers at elevated temperatures. The block copolymers were synthesized through the sequential anionic polymerization of styrene and tert-butyldimethylsilyloxystyrene. The silyl ether protecting groups on the second block were hydrolyzed under acidic conditions to afford poly(styrene-b-4-vinylphenol), and then the pendent phenols of the diblock copolymer were chemically modified with 4-nitrophthalonitrile to afford poly(styrene-b-4vinylphenoxyphthalonitrile). Stable suspensions of similar to 8-10 nm diameter cobalt metal nanoparticles were formed by thermolysis of dicobalt octacarbonyl in solutions of toluene containing poly(styrene-b-4vinylphenoxyphthalonitrile). The cobalt-polymer nanoparticle complexes were pyrolyzed under argon to afford highly magnetic cobalt nanoparticles encased in graphitic-like coatings. Magnetic susceptibility measurements indicate that the cobalt-graphitic particles are oxidatively stable and retain their high saturation magnetizations (similar to 95-100 emu g(-1)) for at least 1 year under ambient conditions.