The reactions between thin refractory metal Ti or Nb films and SiO2 substrates in the temperature range 650–1000 °C in a vacuum have been studied by Rutherford backscattering spectrometry and glancing angle x-ray diffraction. The reactions started with the growth of measureable phases at 700 and 900 °C and completed with the configurations of SiO2 /Ti5 Si3/ TiO and SiO2 /Nb5 Si3 /NbO. Marker experiments indicated that the Ti atoms were the dominant diffusing species in the formation of Ti5 Si3 in the SiO2 /Ti reaction. The formation rate of the Ti5 Si3 layer was thickness dependent: The thicker the original thin Ti film, the higher the rate.