Cathodoluminescence scanning electron microscopy is used to study AlxGa1−xAs epilayers grown on a nonplanar substrate by molecular beam epitaxy. Grooves parallel to the [011¯] direction were etched in an undoped GaAs substrate. Growth on such grooves proceeds on particular facet planes. We find that the aluminum concentration in the epilayers is dependent on the facet orientation, changing by as much as 35% from the value in the unpatterned areas. The transition in the aluminum concentration at a boundary between two facets is observed to be very abrupt.