We investigated the growth of GaAsSb layers and quantum wells (QW) on GaAs (00 1) by MOVPE. Sb concentrations up to 12% were achieved at low arsine partial pressure and low growth temperature. Varying the trimethyl antimony pressure primarily changed the growth rate but not the Sb incorporation. The in situ reflectance anisotropy spectrum during growth resembles that of a GaSb surface. Sb has a strong tendency towards segregation and is only incorporated after reaching a certain critical surface coverage. These findings can be explained by a very mobile and highly Sb enriched surface layer which forms by surface melting due to strain. (c) 2004 Elsevier B.V. All rights reserved.