The diffusion and precipitation of several ion implanted impurities in amorphous Si have been observed at temperatures of 300–600 °C. Typical slow diffusers in crystalline Si, such as As, In, Sb, and Bi, show little or no diffusion at low concentrations. At high concentrations (>1 at. %), they diffuse rapidly with D≳10−15 cm2/s in the temperature range 500–600 °C. Typical fast diffusers in crystalline Si, such as Cu and Au, diffuse in amorphous Si with D>10−12 cm2/s at 400–600 °C. Precipitation has been observed for both the fast and slow diffusers in amorphous Si.