A cluster glass structure has been observed to occur in nanohybrids consisting of zinc ferrite in an amorphous silica matrix. Such a unique cluster glass structure has been supported by studies of x-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and magnetic characterization. By increasing the Fe/Zn ratio in silica gel to 10, crystallites of zinc ferrite of partially inverted structure were nucleated and grown in amorphous iron oxide pockets dispersed in the silica network. The zinc ferrite crystallites, together with the confinement of silica matrix, suppress nucleation and crystallization of α-Fe2O3 to a temperature above 900 °C, preserving the cluster glass structure. The resulting cluster glass structure consists of amorphous iron oxide pockets of ∼ 10 nm in dimensions together with zinc ferrite crystallites of ∼ 3 nm in sizes, which are uniformly dispersed in the amorphous silica glass matrix.© 2001 American Institute of Physics.