We present a new system that allows us to modulate the catalytic activity of metal nanoparticles (Ag) by a thermodynamic transition that takes place within the carrier system. Thermosensitive core-shell particles have been used as the carrier system in which the core consists of poly(styrene) (PS), whereas the shell consists of a poly(N-isopropylacrylamide) (PNIPA) network cross-linked by N,N'-methylenebisacrylamide (BIS). Immersed in water, the shell of these particles is swollen. Heating the suspension above 32 degrees C leads to a volume transition within the shell that is followed by a marked shrinking of the network of the shell. The maximum degree of swelling can be adjusted by the degree of cross-linking. Silver nanoparticles with diameters ranging from 6.5 to 8.5 nm have been embedded into thermosensitive PNIPA networks with different cross-linking densities. The Ag nanoparticles do not influence the swelling and the shrinking of the network in the shell. The surface plasmon absorption band of the nanoparticles is shifted to higher wavelengths with temperature. This is traced back to the varying distance of the nanoparticles caused by the swelling and the shrinking of the shell. The catalytic activity is investigated by monitoring photometrically the reduction of 4-nitrophenol by an excess of NaBH4 in the presence of the silver nanocomposite particles. The rate constant kapp was found to be strictly proportional to the total surface of the nanoparticles in the system. Moreover, kapp is first decreasing with increasing temperature when approaching the volume transition. This is due to the strong shrinking of the network. Only at temperatures above the volume transition is the normal Arrhenius-type dependence of kapp found again. In this way, catalytic activity of the metal nanoparticles enclosed in a "nanoreactor" can be modulated by volume transition over a wide range.