We have prepared polymer core-shell fibers containing salts of palladium, rhodium, and platinum. These salts were reduced within the core of the fibers by thermal decomposition at elevated temperatures and by using hydrogen as reduction medium. Monometal nanoparticles as well as mixed bimetal nanoparticles were produced in this way. The resulting fibers consisted either of a hollow core carrying the nanoparticles or a polymer core in which the nanoparticles were distributed. Both systems were successfully applied to specific hydrogenation reactions in which the shell served the purpose of immobilizing the catalyst nanoparticles, controlling the access to them, and allowing their recovery after the reaction whereas the core acted as the confined reaction site with an easy access of the reactants to the catalytic sites.