GaSb quantum rings in GaAs were studied by cross-sectional scanning tunneling microscopy. The quantum rings have an outer shape of a truncated pyramid with typical lateral extensions between 10 and 30 nm and heights between 1 and 3 nm, depending on the molecular beam epitaxy growth conditions. A clear central opening of varying diameter and more or less conical shape, filled with GaAs, is characteristic for the GaSb rings. The self-organized formation of quantum rings during the growth and subsequent fast overgrowth of GaSb quantum dots is attributed to a combination of large strain with strong Sb segregation. The latter is enabled by extensive group-V atomic exchange reactions at the GaSb/GaAs interfaces, which are quantitatively evaluated from the atomically resolved microscopy data. (C) 2008 American Vacuum Society.