We present the first principles studies on the thermodynamics of several semiconductor alloys using ab initio molecular dynamics simulations. We show that the enthalpy of formation of these alloys can be reliably calculated. For GaAs, both the calculated enthalpy of formation in the temperature range between 300 and 700 K and the optimized crystalline structure are in excellent agreement with the experiments. Results on the thermochemical processes of the formation of several semiconductor materials are also presented. (c) 2005 Elsevier B.V. All rights reserved.