We present a study on the catalytic activity of palladium nanoparticles in different colloidal carrier system, namely in spherical polyelectrolyte brushes (SPB) and core-shell microgels. The SPB carrier particles consist of a solid core of poly(styrene) onto which long chains of poly((2-methylpropenoyloxyethyl) trimethylammonium chloride) (PMPTAC) are grafted. These positively charged polyelectrolyte chains form a dense layer on the surface of the core particles which binds the divalent PdCl42- ions. Reduction leads to metallic Pd-particles. The second system, the core-shell microgels consists of a solid core of poly(styrene) and a shell of crosslinked poly(N-isopropylacrylamide) (PNIPA). The metal-ions were strongly localized within the network because of complexation of the PdCl42- ions and the nitrogen atoms of PNIPA. Reduction of these ions within the polymer layer leads to nearly monodisperse nanoparticles of metallic palladium. The average diameter d of the particles in both carriers is of the order of 2 to 4 nm. Both types of composite particles exhibit an excellent colloidal stability. The catalytic activities of the Pd-nanoparticles were investigated by monitoring photometrically the reduction of p-nitrophenol by an excess of NaBH4. The catalytic activity of the palladium nanoparticles are strongly influenced by carrier systems: The measured rate constants of Pd-nanoparticles immobilized in spherical polyelectrolyte brushes is much higher than the one measured for Pd-particles in the network of the microgels. Additional studies demonstrated that these differences may solely be traced back to the different diffusional barriers in both carrier systems. All data demonstrate that both systems present “smart” carrier systems for metallic nanoparticles: They allow us to synthesize and to immobilize these particles in a well-defined manner. Moreover, the propterties as e.g. the catalytic activity of these nanoparticles can be adjusted in these systems by external triggers as e.g. temperature or pH.