The virtual crystal approximation has been used to determine the temperature-composition dependence of the GaN-AlN, GaN-InN, and InN-AlN band gap energies. Also, the thermodynamic instability states in the mixed crystals were studied. The expression for the band gap of mixed A-B crystals has been derived: Eg
AB = (1−x)Eg
A + xEg
B − bSxx, where Eg
A and Eg
B are the direct gaps for compounds A and B, respectively, and x is the alloy concentration. The term Sxx ~ T0/(∂2G/∂x2) where G is the thermodynamic potential of the mixed crystal, b is a bowing parameter, and T0 has the meaning of a growth temperature.