In the present work we show how the role of ad-atoms and surface imperfections play a key role during the coalescence of thiol-passivated Au nanoparticles (NPs) below 4 nm in diameter. Using spherical aberration corrected scanning transmission electron microscopy (STEM) with ultra-high resolution and computer simulations we reveal the main structural and energetic changes throughout the diffusion/migration of surface atoms during coalescence of monolayer-protected NPs. The sintering process of nanoparticles with similar size (diameter) follows a neck-formation mechanism. However, in the case of thiol-protected Au NPs with different size (i.e. with dissimilar surface curvature) the Ostwald ripening mechanism has been detected with atomic resolution in situ STEM measurements for the first time. In particular we have observed how the larger particles grow in size at the expense of the smaller ones, until the latter completely disappear. The experimental findings have been supported with the aid of state-of-the-art computer simulations. (C) 2012 Elsevier Ltd. All rights reserved.