A polymer-supported spin trap, poly[N-(p-vinylbenzylidene)-tert.-butylamine N-oxide] (polyPBN), was synthesized by an improved procedure. Its copolymers with styrene or methyl methacrylate (copolyPBNs) were also prepared, and their spin-trapping abilities were investigated. Thus, several free radicals generated by the thermal decomposition of radical precursors reacted with the solid polymers dispersed in p-xylene and heptane. Both polyPBN and copolyPBN in the solid state can capture carbon- and oxygen-centered radicals in the heterogeneous phase (solid-liquid system).