Laboratoire de Minéralogie et de Cosmochimie du Muséum, Muséum National d’Histoire Naturelle, 57 rue Cuvier, Paris, 75005, France

The isotopic ratio of hydrogen was measured in an iron meteorite and terrestrial native iron using an IMS 3f ion microprobe. The extraterrestrial D/H ratio (93 ± 9 × 10^-6) was close to the terrestrial value (105 ± 6 × 10^-6), and both samples had low H concentrations (7 ± 4 and 33 ± 11 ng g^-1 for the iron meteorite and the terrestrial sample, respectively). Experiments on artificially D-enriched samples showed that the measured hydrogen signal is a combination of indigenous H and terrestrial atmospheric contamination. This contamination comes from the isotope exchange reaction between water adsorbed on the sample surface and atmospheric water, and would be continuously added to the indigenous H in the ion crater by the adsorbed water sinking into the crater during sputtering. Experiments showed that this contamination represents up to 20% of the signal but was within the uncertainty of the measured D/H ratio.