  Sets of systematic laboratory experiments are presented -- combining Ultra
High Vacuum cryogenic and plasma-line deposition techniques -- that allow us to
compare H/D isotopic effects in the reaction of H2O (D2O) ice with the hydroxyl
radical OD (OH). The latter is known to play a key role as intermediate species
in the solid-state formation of water on icy grains in space. The main finding
of our work is that the reaction H2O + OD --> OH + HDO occurs and that this may
affect the HDO/H2O abundances in space. The opposite reaction D2O + OH --> OD +
HDO is much less effective, and also given the lower D2O abundances in space
not expected to be of astronomical relevance. The experimental results are
extended to the other four possible reactions between hydroxyl and water
isotopes and are subsequently used as input for Kinetic Monte Carlo
simulations. This way we interpret our findings in an astronomical context,
qualitatively testing the influence of the reaction rates.
