This paper describes the features of the boundary in the plasma ion composition near Mars which separates the region dominated by the solar wind protons from the plasma of planetary origin. This boundary was detected by ASPERA experiment on Phobos 2. It is argued that the features of this boundary seem to be similar to those of other composition boundaries detected elsewhere: the cometopause near comet Halley, and a boundary in the ion composition which appears near Venus during periods of high solar wind dynamic pressure. Numerical modeling of the solar wind interaction with Mars supports the idea that during solar maximum the interaction of the Martian neutral atmosphere with the solar wind can result in a composition transition from solar wind to planetary ions in the low-altitude magnetosheath. This transition occurs because of charge exchange of solar wind protons with the neutral atmosphere and photoionization. ¿ American Geophysical Union 1991