Layers of InN and GaN have been successfully grown on GaAs(001) and GaP(001) substrates using a modified molecular beam epitaxy technique. X‐ray diffraction data reveals that these layers are, in fact, separated into binary components of GaAs and mixtures of GaN in its hexagonal or zinc‐blende form. The same composite layers have also been studied with scanning electron microscopy, and we find a close correlation between the grain size measured from the scanning electron microscopy images and those determined from the full width at half‐maximum of the x‐ray diffraction spectra. This key observation demonstrates for the first time that there is a direct relationship between the measured x‐ray half‐widths and the microstructure of the films for both hexagonal and zinc‐blende polytypes. In addition, we have been able to control the growth conditions of GaN such that we can now selectively grow either exclusively hexagonal or zinc‐blende material.