  We have developed the first gas-grain chemical model for oxygen fractionation
(also including sulphur fractionation) in dense molecular clouds, demonstrating
that gas-phase chemistry generates variable oxygen fractionation levels, with a
particularly strong effect for NO, SO, O2, and SO2. This large effect is due to
the efficiency of the neutral 18O + NO, 18O + SO, and 18O + O2 exchange
reactions. The modeling results were compared to new and existing observed
isotopic ratios in a selection of cold cores. The good agreement between model
and observations requires that the gas-phase abundance of neutral oxygen atoms
is large in the observed regions. The S16O/S18O ratio is predicted to vary
substantially over time showing that it can be used as a sensitive chemical
proxy for matter evolution in dense molecular clouds.
