The use of nonequilibrium annealing approaches can produce very high levels of arsenic electrical activation in Si. However, subsequent thermal treatments between 500 and 800°C800 easily deactivate the dopant to a level one order of magnitude below the solid solubility. In this work, the authors study the deactivation of laser annealed (LA) ultrashallow arsenic distributions in silicon using Hall effect measurements, extended x-ray absorption fine structure spectroscopy, and secondary ion mass spectrometry. Single crystal Si (100) wafers implanted with As ions at 2keV2 energy and different doses were activated with a millisecond LA at 1300°C1300 using a scanning diode laser annealing system under nonmelt conditions. The samples were then thermally treated in a furnace at 300–900°C300 in a N2N atmosphere for 10min10. Electrical deactivation has been observed for all the implanted doses but for the lowest one. In particular, it was observed that the higher the As dose the easier the deactivation, in particular, after the 700°C700 post-LA treatment. At 900°C900, in-depth diffusion and a resulting reactivation has been observed for samples implanted with 1×10151 and 3×1015cm−23.