The potential-induced surface reconstruction of Au(100) has been studied by a combination of density functional theory and thermodynamic considerations. Surface free energies of reconstructed-(5 x 1) and unreconstructed-( 1 x 1) surfaces were calculated as function of an external electricfield using the extended ab initio atomistic thermodynamics approach. After relating electric field and electrode potential by using capacitance measurements, we calculate lifting of the reconstruction to occur at 0.58 V in 0.01 M HClO4 and 0.27 V in 0.01 M H2SO4, being in agreement with the experimental values of 0.60 and 0.27 V ( vs. SCE). Finally, the consequences of using experimental capacitance measurements for calculating surface free energies are discussed. (C) 2008 Elsevier B.V. All rights reserved.