One striking observation made during the Phobos-2 mission concerns the existence of a welldefined plasma boundary where the solar wind proton flux terminates. Different mechanisms have been proposed to explain this interesting new feature. In the present study, we explore the possible influence of charge exchange on the loss of solar wind protons in the Martian exosphere as the reason of such a composition boundary. Unlike the cometary interaction, the charge exchange process is effective only in the vicinity of the subsolar point of the Martian "magnetopause" where the streamlines cross the dense neutral atmosphere. The most recent model of the Martian upper atmosphere and the flow field obtained by numerical simulation are used to provide the basis for such a scenario.