The D/H ratio and the 18O/16O ratio in hydronium ions in the gas coma of comet Halley have been determined from ion data obtained by the high intensity sensor (HIS) of the ion mass spectrometer (IMS) on board the spacecraft Giotto in 1986. The HIS instrument measured reliable data for the water group ions over a large distance range from the nucleus. We could therefore minimize the statistical errors. This large range also allowed us to study the systematic influence of formaldehyde on the D/H ratio in the hydronium ion and to deduce from the hydronium ion a D/H ratio for the water molecule. The measured 18O/16O ratio of (1.93¿0.12)¿10-3 is compatible with the telluric value of 2.06¿10-3. The D/H ratio for the hydronium ion measured is D/H=3.08+0.38-0.53¿10-4. We show that since the water chemistry is not significantly different for deuterated and nondeuterated water molecules and ions and since the contribution from other deuterated molecules to the deuterium abundance in the hydronium ion is small, the D/H value of the neutral water molecule is not significantly different from the value determined for the ion, although the uncertainty is somewhat larger. An upper limit for the deuterium abundance in formaldehyde in comet Halley can be derived from the D/H ratio in the hydronium ion as a function of distance from the nucleus. From our results we conclude that the D/H ratio in formaldehyde is certainly smaller than 2% and probably smaller than 0.5% and that the enrichment of the deuterium in the hydronium ion due to formaldehyde is lower than 5%. The D/H ratio in water in comet Halley is therefore significantly higher than the telluric sea water value, whereas the deuterium enrichment in formaldehyde is not as high as, for example, that observed in the Orion compact ridge. ¿ American Geophysical Union 1995