The structural properties of highly strained buried InxGa1-xAs layers on GaAs substrates are investigated by high-resolution x-ray diffraction. Such layers of a few monolayers in thickness serve for the formation of self-organized quantum dots by the Stranski–Krastanow growth mode. Exceeding a critical layer thickness the growth mode changes from two-dimensional Frank–van der Merwe to the three-dimensional Stranski–Krastanow mode resulting in the formation of coherently strained InxGa1-xAs islands. X-ray spectra of such structures below the growth mode transition can be perfectly simulated using dynamical theory allowing for determination of layer thickness with submonolayer sensitivity and composition within 5%. Dot formation manifests itself in a decrease of the effective In content of the wetting layer.