The deuterium flow q p e r m (HD) through a 0.15mm thick stainless steel membrane at 200 o C into vacuum was measured. Deuterium entered the membrane by dissociative adsorption of heavy water (D 2 O) vapour. The q p e r m (HD) at 200 o C caused by this reaction was measured for two equal membranes; for the first a q p e r m (HD) (200 o C) =<1.3x10 6 moleculess - 1 cm - 2 at the partial pressure p(D 2 O)=134mbar was found while a q p e r m (HD) (200 o C) =<3.2x10 6 moleculess - 1 cm - 2 at p(D 2 O)=168mbar was found for the second. This flow at 200 o C was small compared to the residual hydrogen flow q(H 2 ) from the bulk, (q p e r m (HD)/q(H 2 ) <6%). Both q(H 2 ) and q p e r m (HD) were thus orders of magnitude lower than the atmospheric hydrogen permeation rate q p e r m (H 2 ) frequently predicted by existing models, applied to explain the residual q(H 2 ) after particular thermal processing of a ultrahigh vacuum chamber made of austenitic stainless steel. So far, no clear evidence of the relative importance of these two sources has been reported at this temperature. The q p e r m (D 2 ) was not observed in this experiment.