The neutral mass spectrometer experiment (NMS) carried by the Giotto spacecraft was designed to determine the abundances and the chemical, elemental and isotopic composition of the gases and low-energy ions in the coma of comet Halley. Its first results show the predominance of water vapour (∼80% by volume). Preliminary evaluation of the data yields an H2O density of 4.7 × 107 molecules cm−3 at 1,000 km, a neutral gas expansion velocity of 0.9 km s−1 and a total gas production rate of 6.9 × 1029 molecules s−1. The photodestruction scale length for H2O was 3.9 × 104 km. Limits on the abundances of CO2, NH3 and CH4 relative to H2O are given. The contact surface was located at 4,700±200 km, as observed from a change in ion temperature. The water-group ions H3O+, H2O+ and OH+ have been unambiguously identified, along with the ions 12C+, 12CH+, 16O+, Na+, 12C2+, 32S+, 34S+ and 56Fe+. The 34S+/32S+ ratio is 0.045±0.010. General features of the low-energy ion flow at large distances from the nucleus are presented.