In this paper, the formation enthalpies of the final phases presented in annealed contacts composed of transition metals (Pd, Pt, Ni, Ir, Rh, Co, Ti, Cr, V, Zr) and GaAs are calculated by using Miedema's semi-empirical model. Furthermore, the free energies of reaction both for the formations of the final phases and for the decompositions of the final phases containing As are calculated. These calculated results are compared with the experimental values. We reach the conclusion that for most of the above systems, the free energies of reaction can be adopted to assess the formation temperatures and the stabilities for the final phases.