This paper reviews experimental results of phase separation and atomic long-range ordering in A1GaInN alloys, grown by plasma-assisted MBE. Phase separation has been observed in InxGal-x alloys and atomic ordering has been observed in AlxGa1-xN alloys, using XRD studies. Specifically, we find that alloys with In content in excess of 30% show an extra diffraction peak corresponding to an alloy with high In concentration (close to pure InN). The effect of phase separation on the optical properties of the films was also investigated by transmission and photoluminescence measurements. From these studies, we find that the degree of phase separation depends on the thickness of the alloy layer. Atomic long-range ordering has been observed in AlxGal-xN alloys over the entire alloy composition. The phenomenon was investigated by studying the superlattice peaks (0001), (0003) and (0005). The relative intensity of these peaks was found to be largest for the Al content in the 40–50 % range in qualitative agreement with expectations for an ordered structure of ideal Al0.5Ga0.5N stoichiometry. We found that the ratio of III/V fluxes has a significant effect on the degree of ordering.