Nanocomposite made of 10wt% of Co 2.4 Al 0.6 O 4 particles dispersed in an amorphous SiO 2 matrix has been synthesized by a sol–gel method. X-ray diffraction, transmission electron microscopy and magnetic measurements have been used to characterize the properties of nanocomposite. Most of the particles are well crystallized and have an average diameter below 100nm. Smaller particles with size below 10nm have also been observed. A large value of the effective magnetic moment per Co 2+ ion of 5.08μ B and negative and the low Curie–Weiss paramagnetic temperature Θ∼−6K, obtained from the high-temperature susceptibility data, indicate a possible mixing of Co 2+ and Co 3+ ions between tetrahedral and octahedral sites of the spinel crystal lattice. The measurements of static and dynamic magnetic susceptibilities have shown that Co 2.4 Al 0.6 O 4 particles in SiO 2 matrix display a spin glass behavior at low temperatures.