Nanocrystalline transition-metal ferrites were synthesized inside an amorphous silica matrix by the sol-gel method. The formation of spinel ferrites began above 400degreesC, giving fine particles of about 10 nm at 800degreesC. This is associated with a specific role of the silica matrix, which facilitates the diffusion of the reacting cations, enhancing the ferrite formation. Above 1000degreesC the MnFe2O4 and CuFe2O4 nanoparticles lost their fine nature. The dried gels and crystalline materials were characterized by X-ray diffraction, thermal, FTIR, and BET analyses as well as by high-resolution scanning transmission electron microscopy.