The diffusivity and solubility of Cu impurities have been measured in different structural states of amorphous Si (a‐Si) formed by MeV Si implantation. The 2.2‐&mgr;m‐thicka‐Si layers were first annealed (structurally relaxed) at 500 °C and then implanted with 200 keV Cu ions, returning a 300‐nm‐thick surface layer to the as‐implanted state. After diffusion at temperatures in the range 150–270 °C, we observe solute partitioning at a sharp phase boundary between the annealed and Cu‐implanted layers, the partition coefficient being as large as 8.2±1.3. The diffusion coefficient in annealeda‐Si is 2–5 times larger than in as‐implanteda‐Si, with activation energies of 1.39±0.15 and 1.25±0.04 eV, respectively. The data show quite strikingly the role which defects can play in thea‐Si structure.