Abstract To study the influence of the growth parameters on structural properties and crystal quality epitaxial layers of AlN and Al x Ga 1− x N ( x=0–1) were grown in an RF-plasma enhanced MBE-system on Si-terminated (0 0 .1) SiC, (0 0 .1) sapphire and (1 1 1) Si substrates by varying substrate temperature, growth rate and III/V ratio. The alloy composition of the Al x Ga 1− x N layers was studied by RBS-measurements and the results were compared with X-ray diffraction and with the optical properties obtained from cathodoluminescence (CL). The formation of an Al x Ga 1− x N superstructure is observed with ordered Al and Ga distribution. Increasing degree of chemical ordering is observed with increasing growth temperature. The CL-spectra at 77 K show relatively sharp near-band-edge transitions for pure AlN and Ga-rich compositions indicating high material quality. The near-band-edge transition follows the composition of the solid solution connected with a broadening of the peaks and evident optical bowing for samples grown at low growth rates (<0.35 μm/h). A decrease of the bowing parameter is observed for the entire range of alloy composition by increasing the growth rate.