Composition measurements in an accelerated flow event at the Earth's dayside magnetopause show evidence for reflection and transmission of magnetospheric and magnetosheath ion species. Furthermore, a single velocity transformation approximately tangent to the magnetopause surface orders the individual transmitted and reflected ion distributions on both sides of the magnetopause into field-aligned flow at VA, the local Alfven speed. These observations provide strong evidence for a kinetic description of magnetic reconnection at the magnetopause. ¿ American Geophysical Union 1991