Previously reported annealing effects on the carrier density and free‐carrier absorption are correlated with photoluminescence and localized vibrational mode infrared absorption measurements of annealed samples of heavily doped GaAs: Si. Annealing in the range 400≤TA≤750 °C produces the following qualitative changes: (i) a major reduction in the free‐carrier concentrationne, (ii) an increase in the carrier absorption cross section, (iii) a reduction of the SiGalocalized mode absorption band, and (iv) the introduction of a new photoluminescence band at 0.98 eV. A defect model is proposed which is consistent with the observed changes. There is a major reduction in [SiGa], the Si donor concentration, which is a function of the anneal temperature between 400 and 750 °C. The reduction is probably through the formation of (SiGa−VGa) pairs. There is little change if any in [SiAs] the Si acceptor concentration. When TA= 400°C thene= [SiGa] −[SiAs]. WhenTA= 600 and 750°C, a new acceptor is formed having concentrations ≃3 to 4×1018cm−3, in samples having [Si]≃4×1019cm−3, andne≠[SiGa] − [SiAs]. The new acceptor defect is a Si complex with an acceptor state ≃0.54 eV above the valence band. There is no evidence for SiGa→ SiAssite transfer after annealing at anyTAin the 400–750°C range. Annealing at 1100 °C removes all the lower‐temperature annealing effects including the new acceptors and againne=[SiGa] − [SiAs].