A key problem in unfolding the physics of magnetospheric boundary layers is that of clearly identifying ion source and transport characteristics. Direct measurements of ion composition near the magnetopause have recently demonstrated the existence of the ionospheric species O+ and He+ as well as the solar wind species He++ within the magnetospheric boundary layer. Based on a selected group of 13 boundary layer crossings by the AMPTE/CCE spacecraft and using the Charge-Energy-Mass Spectrometer (CHEM) of the University of Maryland and Max-Planck-Institut f¿r Aeronomie/Lindau, we have clearly identified several additional ion species that overlap the boundary layer. Ion species of ionospheric origin such as O+, N+, He+, and the molecules NO++O+2, as well as solar wind ions such as He++ and the high charge-state groups of CNO, Si, and Fe, are found in the frontside, low-latitude boundary layer. By comparing ratios of ion number densities observed in each of the 13 crossings we find that ion densities in the boundary layer are roughly one-half those observed in the respective source regions. On average, only 1 to 9% of ions are found in the region on the other side of the boundary layer away from their respective source region. Our observations indicate that such transport across the boundary layer is a one-way process with dawn-dusk asymmetry. Individual ion density ratios from which these various averages are obtained cover a very wide range of values indicating large spatial or time variability. These results are not consistent with any simple steady state process for producing the boundary layer; however, our analysis to date cannot distinguish between possible non-steady state processes such as impulsive penetration or non-steady reconnection. In summary, the boundary layer is a region of overlap of ions of both solar wind and magnetospheric origin with high variability in ratios of ion densities. ¿American Geophysical Union 1990