The reactions between thin refractory metal Ti or Nb films and SiO2substrates 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/Ti5Si3/ TiO and SiO2/Nb5Si3/NbO. Marker experiments indicated that the Ti atoms were the dominant diffusing species in the formation of Ti5Si3in the SiO2/Ti reaction. The formation rate of the Ti5Si3layer was thickness dependent: The thicker the original thin Ti film, the higher the rate.