Infrared absorption bands of localized vibrational modes for ion‐implanted Si in GaAs are observed. Room‐temperature implants were at energies of 1.0 and 1.5 MeV and fluences ∼ 1017 ions cm-2. The bands previously attributed to SiGa and SiAs defects were observed and the dependence of the absorption at liquid‐nitrogen temperature was studied for samples isochronally annealed from 200 to 900 °C. The strongest band due to (SiGaSiAs) pairs was observed after 400 °C annealing. For most of the temperature range the different defect concentrations were found to either grow or show little change during the annealing cycle. However, between ∼ 500 and 650 °C substantial reverse annealing was observed for both SiGa and SiAs. This effect cannot be explained by direct pair formation and other possible explanations are suggested. For these high‐fluence, high‐energy implants the electrical data obtained during the anneal cycle show no apparent correlation with the Si defect concentrations as deduced from optical data. The conductivity is probably controlled by residual damage left in the implant layer.