Isotopically enriched water, [18O] H(2)O, recovered from the synthesis of [18F] fluorine-labelled PET radiopharmaceuticals, is frequently diluted with natural water, [16O] H(2)O. The extent of isotope dilution limits the usefulness of the recovered water. A simple method for determining the [18O] enrichment of water mixtures could aid PET cyclotron facilities to calculate the [18F] yields to be expected when the recovered [18O] H(2)O is reused. A gravimetric method (repeatability for high enrichment +/-0.3% [18O], detection limit approximately 2% [18O] enrichment) is described.