A sharp (≤104 km thick) transition from a solar wind proton dominated flow to a plasms population primarily consisting of relatively cold cometary heavy ions has been observed at a cometocentric distance of about 1.6¿105 km by the VEGA and GIOTTO missions. This boundary (the cometopause) was though to be related to charge transfer processes, but its location and thickness are inconsistent with conventionally estimated ion-neutral coupling boundaries. In this paper, a two-fluid model is used to investigate the major physical processes at the cometopause. By adopting observed comet Halley parameters the model is able to reproduce the location and the thickness of this charge exchange boundary. ¿ American Geophysical Union 1987