The interaction of the solar wind with cometary plasma is studied in the framework of a collisionless 2D bi-ion fluid plasma model which allows for different bulk velocities of the light and heavy ions. It is shown that an ion composition boundary is formed which is impenetrable for the protons but presents no barrier to the magnetic field flux. The magnetic field is transported further downstream by the charge-neutralizing electrons which follow the heavy ions through the boundary. We believe that this new type of plasma boundary can explain the observed ion composition boundary at comets (cometopause, pile-up boundary), at Mars (planetopause, massloading boundary) and at Venus (rarefaction wave, intermediate transition). ¿ American Geophysical Union 1994