Thermoelastic properties are important for modeling thermal residual stresses and for optimizing the growth conditions of semiconductor thin films. Thermal expansions of AlN and GaN have been evaluated and predicted by us earlier [1][2]. Here, high temperature elastic constants are estimated empirically from corresponding state relationships and data from other hexagonal Grimm-Sommerfeld compounds. This information together with our earlier thermal expansion data will further improve capabilities for calculating thermal residual stresses in various semiconductor thin films.