Transition metal oxide nanoparticles were synthesized within mesoporous MCM-48 silica phases. By wet impregnation, drying, and calcination procedures, it was possible to form iron, iron/cobalt, cobalt, nickel, and copper oxide nanoparticles almost exclusively within the pore system. Nitrogen sorption measurements still revealed mesoporosity for the host/guest compounds accompanied by a reduction of the surface area and pore radius, both indications of a decoration/coating of the inner surface of the silica walls. X-ray absorption spectroscopic measurements proved the existence of small, slightly disordered metal oxide nanoparticles. HRTEM investigations showed that the mesoporous host structure was still intact after the treatment. Thermogravimetric investigations revealed differences in the reduction/oxidation behavior of the metal oxide nanoparticles compared to their corresponding bulk phases.