Observations in the coma of P/Halley by the Giotto ion mass spectrometer (IMS) are reported. The high-energy range spectrometer (HERS) of the IMS obtained measurements of protons and alpha particles from the far upstream region to the near ionopause region and of ions of mass 12--32 at distances of about 250,000 to 40,000 km from the nucleus. Plasma parameters from the high-intensity spectrometer (HIS) of the IMS obtained between 150,000 and 5000 km from the nucleus are also discussed. The distribution functions of water group ions (''water group'' will be used to refer to ions of 16--18 m/q, where m is in atomic mass units and q is in unit charges) are observed to be spherically symmetric in velocity space, indicating strong pitch angle scattering. The discontinuity known as the magnetic pileup boundary (MPB) is apparent only in Proton, alpha particle, and magnetometer data, indicating that it is a tangential discontinuity of solar wind origin. HERS observations show no significant change in the properties of the heavy ions across the MPB. The issue of whether a cometopause was unambiguously observed at comet Halley is discussed; it is concluded that the observations do not convincingly support the idea of a boundary due to internal cometary processes (as opposed to convected solar wind discontinuities). A comparison of the observations to MHD models is made. The plasma flow directions at all distances greater than 30,000 km from the nucleus are in agreement with MHD calculations. However, despite the agreement in flow direction, within 200,000 km of the nucleus the magnitude of the velocity is lower than predicted by MHD models, and the density is much larger (a factor of 5 or more for distances 60,000--160,000 km from the comet). Within 30,000 km of the nucleus there are large theoretical differences between the MHD model flow calculations for the plane containing the magnetic field and for the plane perpendicular to the magnetic field. The observations agreed much better with the pattern calculated for the plane perpendicular to the magnetic field. The data obtained by the HERS that are published herein have also been provided to the International Halley Watch archive. ¿ American Geophysical Union 1992