The redistribution and electrical activity of implanted arsenic in separation by implanted oxygen (SIMOX) and bulk silicon substrates are compared. The SIMOX substrates were prepared by high dose oxygen implantation followed by high-temperature annealing. Subsequently doses of 5×1015 As+/cm−2 at 70 keV were implanted into the substrates and, also, bulk silicon. The samples were annealed to activate the arsenic. It was observed that (i) the same percentage activation was measured in both SIMOX samples and bulk silicon, (ii) no anomalous diffusion was seen in the SIMOX samples, and (iii) during further high-temperature annealing the buried oxide acts as a diffusion barrier. This data confirms the high quality of SIMOX material currently being prepared.