We have used a 90° double reflection high-energy electron diffraction (RHEED) setup to perform a comprehensive real-time study of the morphology of vicinal GaAs(001) surfaces during molecular beam epitaxy. The technique allows to record RHEED intensities simultaneously in the [1¯10] and [110] azimuths and thus enables a detailed study of anisotropy effects. Comparative measurements on surfaces with 2° misorientation towards (111)Ga (A surface) and towards (11¯1)As (B surface), respectively, show that independent on the step type and reconstruction anisotropy, recordings of the specular beam intensity in the azimuth perpendicular to the steps are clearly dominated by the evolution of the staircase order whereas intensity recordings in the azimuth parallel to the steps reveal the evolution of the step edge roughness. Measurements over a wide range of substrate temperatures give insight in the competition between kinetic processes and thermodynamic equilibrium on a length scale accessible to RHEED. For the A surface the transition between two-dimensional (2D) growth and step-flow growth occurs not only at higher temperature than on the B surface, but the disappearance of the intensity oscillations occurs also at different substrate temperatures in different azimuths. The ∼20 °C higher disappearance temperature in the [1¯10] azimuth is explained with a model based on previous scanning tunneling microscopy results which revealed an increasing elongation of the islands in [1¯10] direction with increasing substrate temperature. The B surface is more isotropic and hence no difference in the transition temperature in the two azimuths could be detected. During growth in the transition range between 2D and step-flow growth we observe increased terrace width fluctuations on the B surface, whereas the A surface becomes more uniformly stepped. This demonstrates that in the kinetically controlled regime the anisotropic barrier height for downward diffusion of adatoms over step edges plays an important role for the evolution of the surface morphology. At elevated temperature the barrier height allows downward jumps of the adatoms over B-type steps but not over A-type steps. At conditions close to the thermodynamic equilibrium a kinetic smoothing is observed on the A as well as on the B surface indicating another mechanism to be effective with a change of the energetics due to ordering of the steps in combination with a disordering of the reconstruction on the terraces. This surface is, however, metastable and recovers after growth interruption rapidly (at substrate temperatures >580 °C within less than 1s) to the equilibrium bunched surface. ©1997 American Institute of Physics.