The dynamics of the molecular-beam-epitaxy (MBE) of GaAs (0 0 1) was investigated with a surface X-ray diffractometer coupled to an MBE chamber. The diffracted intensity of X-rays was measured in real time during growth. The temporal oscillation of the intensity for the growth at 550degreesC was explained well by the distributed-growth model, while the intensity for the growth at 435degreesC showed a complicated behavior arising from the changes in the surface reconstruction and the morphology. A method for separating the dynamics of the morphology from that of the reconstruction is presented. (C) 2002 Elsevier Science B.V. All rights reserved.