Isothermal and isochronal annealing measurements were performed on heavily Si‐doped GaAs. Infrared reflectivity measurements were used to determine the free‐carrier concentration after each annealing stage. A factor of [inverted lazy s]5 decrease in free‐carrier concentration was observed as a result of annealing at temperatures as low as 400°C. This annealing effect can be important when fabricating devices using GaAs: Si. Possible explanations of this effect are discussed.