X-ray interference effect, reflection high-energy electron diffraction, and transmission electron microscopy were used to determine the critical thickness of InAs, InP and GaP on GaAs {001} grown by atomic layer molecular beam epitaxy. Three different series of samples consisting in N monolayers of InAs (N=1, 2, 3, 4), M monolayers of InP (M=3, 4, 5, 6, 7) and L monolayers of GaP (L=2, 3, 4, 5, 6, 9) covered by a 200 nm GaAs cap layer were grown at 350°C. The thicknesses of the strained layers were chosen to cover the range from strained layers (below the critical thickness for generation of misfit dislocations) to relaxed layers (where all lattice mismatch is relieved by the generation of misfit dislocations). Sample growth was monitored by reflection high-energy electron diffraction in order to in-situ study the relaxation process. X-ray interference effect was used to determine thickness and strain status of the strained layers comparing experimental diffraction patterns with simulated ones using dynamical theory of X-ray diffraction. Transmission electron microscopy was used to assess thickness, relaxation status and dislocation nucleation in the strained layer. © 1993.