X-ray diffraction is used for the in situ characterization of the growth of cubic GaN by metalorganic chemical vapor deposition. Our setup permits the simultaneous measurement of a wide angular range and requires neither goniometer nor exact sample positioning. Time-resolved measurements during growth give access to film thickness and growth rate as well as information on the chemical composition of ternary compounds. Additionally, the relaxation of the crystal lattice during heteroepitaxial growth of GaN on AlGaN can be measured directly. (C) 2004 American Vacuum Society.