The lattice parameter of AlAs has been very accurately measured as a function of temperature between 15° and 840°C, and the coefficient of thermal expansion has been determined. The lattice parameter at 0°C is 5.6605±0.0005 Å and the coefficient of thermal expansion is (5.20±0.05) ×10−6/°C. A comparison is made to GaAs which indicates that there is a perfect lattice match at about 900°C and 0.14% lattice mismatch at room temperature.