Oxygen O** plus ions have been implanted into Si both at room temperature and liquid nitrogen temperature in order to determine the effect of implantation temperature on SiO//2 formation. Samples were analysed by RBS with 1. 5 MeV He** plus ions. The implants of O** plus in Si were done at 150 keV with current densities of less than equivalent to 10 mu A/cm**2 . For doses of more than 1. 5 multiplied by 10**1**8 O** plus /cm**2, in-situ RBS experiments positively indicate a 2:1 oxygen silicon ratio. Increased O** plus doses (for both room temperature and liquid nitrogen temperature) cause the SiO//2 layers to spread uniformly and symmetrically toward both the surface and the interior. From these results, it is apparent that excess oxygen diffuses toward Si/SiO//2 interfaces in our experimental situation even at liquid nitrogen temperature.