Six shallow donors are shown to form in Czochralski silicon deliberately doped with hydrogen during the early stages of heat treatment at 350 degrees C. The ground state energies of five of these donors are altered slightly in material doped with deuterium rather than hydrogen, demonstrating the presence of hydrogen in their cores. One of the donors may account for some of the weak IR absorption features detected in as-grown material. The formation of these donors appears to be linked to the generation of the well known oxygen-related double thermal donors and may reflect the partial passivation of these latter defects.