In situ stress measurements were made during Al underpotential deposition (upd) onto (111)-textured Au from Lewis acidic aluminum chloride, 1-ethyl-3-methylimidazolium chloride (AlCl3-EMImCl), using the wafer curvature method. The surface stress response consists of three distinct features. In the potential range of 1.2-0.6 V the surface stress moves in the tensile (positive) direction from a value arbitrarily chosen as zero. This likely involves the desorption of AlCl4- from the Au surface and is consistent with adsorbate-induced stress models that appear in the literature. At the start of Al upd, the surface stress moves in the compressive direction, in contrast to the tensile stress expected based on the positive lattice misfit. We attribute this compressive stress to the formation of Al-Au bonds which partially satisfy the bonding requirements of the Au surface atoms, thereby reducing the tensile surface stress inherent to the clean Au surface. In the latter stages of Al upd, the surface stress once again moves in the tensile direction, which we attribute to Al-Au alloying. The magnitude of the tensile stress change is close to that estimated from the elastic strain associated with the change in molar volume, using reaction kinetics reported for surface alloy formation in the upd region. (C) 2008 The Electrochemical Society.