The structural properties of highly strained buried In x Ga1−x As 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 In x Ga1−x As 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.