Static phase diagrams for GaAs(001) and AIAs(001) surfaces have been determined using the reflection high-energy electron diffraction technique. The investigated structures have been grown by molecular beam epitaxy with a Riber 32P machine. Two types of observations of surface reconstructions were performed : without any arsenic flux and for variable arsenic flux. In the experiments both the substrate temperature and the As, flux were measured directly (the absolute values of the As, flux are given in the phase diagrams). On the GaAs(001) surface reconstructions (4X2), (3X1), (3X6), (2X4) and (2X2) were observed; on the AIAs(001) surface the reconstructions (3X2), (2X4), (2X2), and (2X3) were seen. The boundaries between different reconstructions have been determined precisely both for the GaAs(001) and the AIAs(001) surfaces. On the basis of the static phase diagrams the values of activation energies for phase transitions on the GaAs(001) and the AIAs(001) surfaces have been found. The obtained results have been compared with those already existing in the literature.