The GaAs(0 0 1)-(2 x 4) reconstructed surface was investigated by in situ surface X-ray diffraction. X-ray diffraction patterns were measured with increasing substrate temperature within the beta phase of GaAs(0 0 1)-(2 x 4) in a constant As flux of 5 x 10(-7) Torr. At relatively low temperatures up to 545 degreesC, the observed X-ray diffraction patterns agree well with the beta2(2 x 4) surface. However, a different X-ray diffraction pattern was obtained at temperatures close to the alpha phase, while the 2 x 4 periodicity still persisted. This change is explained by partial As-dimer desorption which results in a mixture of the beta2(2 x 4) and alpha2(2 x 4) structures. (C) 2004 Elsevier B.V. All rights reserved.