Complete electrical activity was obtained by cw laser annealing of 7×1015 As/cm2 implanted into (100) Si at 100 keV. The peak concentration for these implantation conditions is 1.4×1021/cm3, both theoretically and experimentally. However, this peak concentration was found to be thermally unstable, relaxing to a value of 3×1020/cm3 in a period of less than 2 min at 900 °C. If the peak implanted concentration is below 3×1020/cm3, the electrical activation and crystal structure are unaffected by similar thermal processing. We conclude from these data that the solid solubility of As in Si at 900 °C is approximately 3×1020/cm3, which is almost an order of magnitude below the published value.