A combination of a molecular beam epitaxy (MBE) machine and a six circle diffractometer has been constructed at a dedicated wiggler beamline at the storage ring BESSY II for in situ investigations of III–V compound crystal growth. The growth conditions in our system reach a high MBE standard with a noncooled base pressure of ${\text{2×10}}^{\text{−10}} \mathrm{mbar.}$ A fast entry load lock is available for sample exchange. Large-area Be windows in the ultrahigh vacuum chamber allow us to measure reflections at entrance and exit angles up to $\text{45°,}$ i.e., large perpendicular momentum transfers are possible. In situ reflection high energy electron diffraction and x-ray fluorescence measurements can be performed simultaneously with x-ray scattering. A GaAs(001) surface prepared and examined in our system reveals terrace widths of 450 nm and $\text{β(2×4)}$ reconstruction domain sizes of 210 nm. The possibility of time-resolved x-ray diffraction studies is demonstrated by observation of intensity oscillations during layer-by-layer homoepitaxial growth on the $\mathrm{GaAs}\text{(001)β(2×4)}$ surface. The resolution functions of our experiment are determined.