We useinsitureflection high‐energy electron diffraction to investigate the growth kinetics of cubic (001)‐GaN/GaAs grown by plasma‐assisted molecular beam epitaxy. We find that the GaN surface exhibits three surface reconstructions having (1×1), (2×2), andc(2×2) symmetries, which correspond to Ga adatom coverages of 0, 0.5, and 1, respectively. We demonstrate that the transient behavior of the half‐order streak intensity is a sensitive probe of the surface stoichiometry during growth. Particularly, these measurements enable us to directly determine the effective N flux incorporated into the crystal. ©1996 American Institute of Physics.