Surface structures of GaAs(0 0 1) under molecular beam epitaxy conditions are described on the basis of in situ X-ray diffraction measurements. Exploiting the high-intensity, high angular resolution and element sensitivity of synchrotron X-rays, we revealed the details of the most frequently used surfaces including several phases of (2 x 4) and c(4 x 4). (c) 2006 Elsevier B.V. All rights reserved.