Dispersion and chemical composition changes of a Ni/SiO2 catalyst prepared by sol-gel processing caused by thermal treatment in hydrogen and oxygen atmospheres have been investigated by means of electron microscopy, electron diffraction and Fourier transform infrared (FTIR) spectroscopy of chemisorbed CO. It has been established that reduction of the catalyst at 923 K leads to a strong metal-support interaction (SMSI) and formation of a cubic NiSi-phase. This causes pronounced changes in the FTIR spectra of chemisorbed CO, both in the total intensity and in the ratio of bridged versus linear bonded CO. Oxidation at 823 K and subsequent reduction at 773 K resulted in redispersion of the metal and formation of small Ni particles with a mean diameter of 2.1 nm.