Hydrogen enhancement of formation rates for oxygen-related thermal donors in Si has been investigated for dependence on the source of hydrogen, hydrogen isotope, and exposure time and temperature. Hydrogen injection efficiency is an important variable and depth profiles are dependent upon the surface preparation of samples exposed in an electron cyclotron resonance plasma where ion energies are less than or equal to 35 eV. Formation rates up to 2 x 10(16) cm(-1)/h at 400 degrees C have been observed. A sublinear dependence of the donor formation rate on beam current under 50 keV ion implantation is interpreted as a competition between oxygen-hydrogen and hydrogen-hydrogen interactions. Dependence on isotope mass and on exposure time in the plasma indicates hydrogen is the diffusing species that determines the penetration depth for the enhanced donor formation. Peculiar box-like depth profiles and high formation rates near the advancing front produced in RF plasma exposures are suggestive of hydrogen accumulation near the advancing front. The temperature dependence for the penetration depth gives an activation energy of 1.5 +/- 0.2 eV. This energy is attributed to trap-limited diffusion wherein hydrogen lowers the energy barrier for the oxygen motion necessary to form thermal donors.