Isothermal changes in the electrical resistivity of lithium‐diffused silicon have been measured in the range of 370°–650°C. Silicon with high and low oxygen content was used to study the effect of lithium‐oxygen interaction. The change of the isothermal curve due to the radiation was also studied. Some important observations can be made: (1) There is evidence of the formation of a new defect at high temperature which would explain the observed redegradation in the post radiation recovery in silicon with low oxygen concentration. (2) There is a very slow formation of a donor state in silicon with high oxygen concentration. (3) The final resistivity after a long period of heat treatment is dependent on the initial donor concentration before the diffusion of lithium in the silicon with low oxygen concentration, while in the case of high oxygen concentration the final resistivity is substantially independent of the initial donor concentration.