Electrical measurements and Rutherford backscattering have been used to evaluate the thermal stability of single crystal and polycrystalline Si films that were ion implanted and laser annealed. The films were implanted with 75As or 31p and annealed with either a pulsed ruby, a pulsed Nd:YAG or a CWAr+ laser. The samples were then thermally annealed at temperatures between 450 and 900°C. The single crystal samples implanted with arsenic below 1 × 10-15 cm−2 were thermally stable. For higher doses the electrical concentration reaches a minimum at 800°C. The same trends are observed in the polysilicon films for sufficiently high doping levels. RBS shows that As is precipitating at 700°C in single crystal material and has begun to go back into solution at 900°C for concentrations of ∼7 × 1020 cm−3. Similar trends are observed for 31P implanted samples.