The effects of ion implantation in an MOS structure were investigated. It was found that depending on the choice of implantation parameters, the effects may include: (1) displacement damage in the oxide, (2) creation of fast states and positive fixed charge near the Si/SiO2interface, and (3) doping in the substrate. MOS capacitance measurements, used in conjunction with isothermal annealing, indicate that the fast states are located in the center of the forbidden gap and are most likely physically associated with point defects created by implantation. In O+implanted specimens, a residual positive charge was formed after an annealing treatment which removed the fast states. The residual charge is probably attributable to a reoxidation effect during annealing.