The data show that the deuterium added in water together with synthesis gas provides hydrogen for the Fischer-Tropsch synthesis. In fact, the deuterium initially present in water nearly equilibrates with the hydrogen present in synthesis gas. Thus, water, once formed, is not inert but adsorbs competitively on the cobalt-titania catalyst to activate hydrogen. The data do not permit a definition of whether the exchange occurs on cobalt or the alumina support. The H/D ratio in the paraffin products is 4.4 and is very close to the H/D ratio in the feed of D2O/H2 (4.1).