We have used interfacial force microscopy (IFM) and contact-mechanics analysis to quantitatively determine the deformation behavior and the mechanical properties of nanometer-size grains in polycrystalline Au films on various substrates. We evaluate these properties through an analysis of the loading-cycle curves and through a comparison of surface morphology before and after each nano-indentation measurement by constant repulsive-force imaging. All the Au surfaces were coated with self-assembling monolayers of n-octadecanethiol to passivate the adhesive interaction between the tungsten probe and Au surfaces. Our results show that both the deformation behavior and mechanical properties vary strongly as a function of substrate and these variations are mirrored by changes in such factors as the interfacial adhesion, grain size and shape, and the presence of grain-boundary impurities.