The use of molecular sieve adsorbents for the continuous recovery of H 2 18O from the gas stream of an isotope separation system is complicated by the exchange of oxygen atoms between the water and the adsorbents. About 75 per cent of the oxygen atoms in the molecular sieve adsorbents exchange with H 2 18O in 2 hr at 250°C. At 135°C, only 24 per cent of the oxygen atoms exchange in 64 hr, and at room temperature the exchange is very slow. Carbon dioxide exchanges with about the same fraction of oxygen atoms in the molecular sieve adsorbents as water does. This exchange probably occurs through small quantities of water vapour since, after prolonged drying under high vacuum at 250°C, the rate of exchange with carbon dioxide is somewhat slower. Oxygen gas does not exchange at 250°C with molecular sieve adsorbents in the presence or absence of water-vapour.