A one-dimensional single-fluid MHD model was used by Cravens (1989a) to predict the existence of a narrow (50 km) layer of enhanced plasma density (factor of 3 over background) at the boundary of the comet Halley diamagnetic cavity. The existence of such a layer was confirmed by measurements made by the Giotto ion mass spectrometer (Goldstein et al., 1989), although with a relative density enhancement of more than a factor of 4. We solve the time-dependent coupled continuity equations for several species including H3O+, H2O+, OH+, O+, NH4+, NH3+, CH4+, and CH3+. For several ion species we investigate factors affecting the magnitude of this enhancement and its relationship to the thickness of the transition layer. For example, ion species with short chemical lifetimes are shown to have smaller density enhancements than species with longer lifetimes. The cometocentric distance of the cavity boundary also strongly affects the magnitude of the density enhancement; the enhancement increases with increasing distance. ¿ American Geophysical Union 1990