The fraction of substitutional Te atoms in silicon deduced from temperature dependent Hall measurements is comparable to that derived from channeling experiments. The donor activation energy of these atoms is found to be about 0.19 eV. Approximately 10% of the Te-atoms give rise to a shallow donor level atEc-0.06 eV and is probably associated with a defect. Substitutional iodine is not electrically active in Si up to 450 K. From this a lower limit of the donor activation energy of 0.30 eV is estimated.