Data is presented on a room-temperature instability which has been observed in certain types of MOS systems after bombardment with ionizing radiation. This instability can cause the transfer characteristic of an irradiated MOS transistor to shift rapidly when bias is applied. The shift can be as much as 3 volts for a unit that has been bombarded with 1015 1 MeV electrons/cm2. In order to explain this effect it is postulated that the ionizing radiation introduces a number of surface trapping sites in the oxide which can exchange charge very slowly with silicon. Shifts in transfer characteristic under bombardment, due to immobile charge accumulation in oxide were also observed. The shifts observed were unusual in that they indicate negative charge buildup, which is contrary to the usual finding.