In this work x-ray-diffraction measurements from GaAs surface gratings are presented. The experiments were performed using a three-crystal diffractometer. Measurements in the region of total external reflection (small incidence angles) for five samples were done and compared with model calculations based on a dynamical scattering theory. The theory is able to explain all experiments quantitatively. Mesoscopic grating parameters as well as microscopic surface roughnesses of the samples were obtained from fits of the data. For three samples scanning-electron-microscope pictures were taken. The analysis of these pictures leads to the same mesoscopic parameters as obtained from x-ray diffraction.