Ab-initio total energies imply that about 43% of the D@sub 2@O molecules in a first layer on Ru(0001) optimally dissociate to adsorbed OD+D. This compares favorably to new x-ray photoemission measurements,@footnote 2@ which say that (3±0.25)/8 of them do. In the optimal wetting layer structure, atop sites on roughly 6% of the outer-layer Ru atoms are available as a natural pathway for D-adatom transport. They represent a surface analog of L-type, Bjerrum defects.@footnote 3@ An equal percentage of the wetting layer's H-bonds, disrupted by the presence of a second D atom, represent the corresponding D-type defects. @FootnoteText@ @footnote 1@ Supported by the USDOE, Office of Basic Energy Sciences, Division of Materials Sciences. Sandia is operated by the Lockheed-Martin Company under USDOE Contract No. DE-AC04-94AL85000.@footnote 2@ J. Weissenrieder, A. Mikkelsen, J.N. Andersen and P. J. Feibelman and G. Held (unpublished).@footnote 3@ N. Bjerrum, Kong. Dansk. Vid. Sels.Mat.-fys. Medd. 27, 1(1951).