The unstable composition region of B1-x-yGaxAlyN was calculated from the free energy of mixing by using the strictly regular solution model. The interaction parameter used in this calculation was based on the delta-lattice-parameter method. The proportionality constant K of 1.15 x 10(7) cal/mol Angstrom(2.5) was determined by fitting the spinodal isotherms to the experimental data of B1-xGaxN grown at 1000 degrees C by MOVPE. Using this K value, the interaction parameters of BN-GaN, BN-AIN and AlN-GaN are 70 730, 54 872 and 1091 cal/mol, respectively. The high interaction parameters of BN-GaN and BN-AlN result in very wide unstable composition regions. From the calculated spinodal isotherms, the maximum boron solubility having a uniform single-phase solid solution is 1.8% in B1-xGaxN and 2.8% in B1-yAlyN at 1000 degrees C. However, the GaAlN alloy can be deposited over the entire composition range since the spinodal isotherms are parallel to the GaN-AlN line.