The use of H-passivation as an initial surface for Si MBE or CVD is becoming more widespread. The H-terminated Si(100) surface is prepared using the spin-etch, vapour-cleaning with HF, or an HF dip, as the final pre-clean before sample loading. The hydrogen is then desorbed in-situ prior to deposition, typically at ≈650 °C. We have shown that deposition directly onto these surfaces leads to epitaxy at substantially lower temperatures (≈350°C). In order to understand how epitaxy occurs far below the hydrogen desorption temperature we have investigated growth on clean Si(100) during deliberate dosing with deuterium in this temperature regime. Since the final coverage observed in SIMS is always far lower than the total dose, we conclude that deposition of Si induces desorption of H, as opposed to simple H segregation.