Exchange Monte Carlo calculations in the semi-grand canonical ensemble are used to determine the mixing properties of AIN with GaN and InN in both the wurtzite and zinc blende structures. For InxAl1-xN solid solutions the difference in structure is reflected in the properties of the phase diagrams. The calculated consolute temperature is approximate to 150 K greater for the cubic phase. The calculated phase diagrams for the two structures are significantly asymmetric with the maximum in the binodals lying markedly on the Al rich side. The GaxAl1-xN solid solution is almost ideal and no observable difference between the phase diagrams for the hexagonal and cubic structures. (c) 2007 Elsevier B.V. All rights reserved.