The shape of the velocity distribution of water group ions observed by the Giotto ion mass spectrometer on its approach to comet Halley is modeled to derive empirical values for the rates of ionization, energy diffusion, and loss in the midcometosheath. The model includes the effect of rapid pitch angle scattering into a bispherical shell distribution as well as the effect of the magnetization of the plasma on the charge exchange loss rate. It is found that the average rate of ionization of cometary neutrals in this region of the cometosheath appears to be of the order of a factor 3 faster than the ''standard'' rates ~1¿10-6 s-1 that are generally assumed to model the observations in most regions of the comet environment. For the region of the coma studied in the present work (~1--2¿105 km from the nucleus), the inferred energy diffusion coefficient is D0≈0.002 to 0.005 km2 s-3, which is generally lower than values used in other models. The empirically obtained loss rate appears to be about an order of magnitude greater than can be explained by charge exchange with the ''standard'' cross section of ~2¿10-15 cm2. However such cross sections are not well known and for water group ion/water group neutral interactions, rates as high as 8¿10-15 cm2 have previously been suggested in the literature. Assuming the entire loss rate is due to charge exchange yields a rate of creation of fast neutral atoms of the order of ~10-4 s-1 or higher, depending on the level of velocity diffusion. The fast neutrals may, in turn, be partly responsible for the higher-than-expected ionization rate. ¿ American Geophysical Union 1994