A study of Si detector response has been carried out using beams of monoenergetic (0.5–2.5 MeV) H and4He ions backscattered from a thick gold target, in an attempt to determine if the resulting pulse height could be used as an energy calibration technique. At each chosen energy, the accelerator had been calibrated to within 0.5 kV by means of a suitable nuclear resonance reaction. After applying small corrections for the nuclear (i.e. non-ionizing) energy loss, and for the energy loss in traversing the surface dead layer of the detector, the observed pulse heights are found to be exactly proportional to the energy of the incident H or He ion, within the experimental reproducibility of 0.2%. In the case of4He ions, the linearity of detector response was verified up to 5.48 MeV by means of a standard241Am radioactive source. One strange anomaly is that the energy ϵ to create an electron-hole pair in silicon is significantly smaller for He (and6Li) than for H ions: namely, ϵHe/ϵH= 0.989 and ϵLi/ϵH=0.967. Various mechanisms proposed to explain this difference are not consistent with the observedlineardependence on particle energy. A comparison between H and D ions and between3He and4He ions showed that this difference in detector response was not related to the mass of the incident ion.