Grain boundary stress relaxation in Au polycrystals and single crystals has been studied by mechanical spectroscopy. A relaxation peak related to grain boundaries is observed at 620 K. Molecular dynamics simulations on Au are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak in Au polycrystals. A Sigma 5 grain boundary is submitted to a shear deformation parallel to the boundary plane. In agreement with previous studies on Cu, the grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700 K. Above 1000 K, only grain boundary sliding occurs. Two models are developed that provide expressions for the relaxation strength Delta and the relaxation time tau that are compared to experimental measurements performed on polycrystals. The observed grain size dependence of Delta and tau favours the sliding model over the migration model. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.