Thermal expansion of GaAs and InAs was measured in the temperature ranges from 396 to 1149 and from 441 to 1206 K, respectively. The thermal-expansion coefficients, Debye temperatures, and root-meansquare atomic displacements were calculated. These parameters were found to depend on the root-mean-square atomic weights of the compounds. Based on the published data and our heat capacity measurements on GaAs and InAs, the coefficients appearing in the equation for the temperature dependence of heat capacity in the temperature ranges from 200 K to the melting points of GaAs and InAs were determined