Highly dispersed supported gold offers unprecedented catalytic properties. Determination of the dependence of the catalytic properties on the gold nanocluster size requires the preparation of size-controlled gold nanoclusters on support surfaces with a high degree of uniformity. Starting from site-isolated mononuclear gold complexes on high-area MgO, we demonstrate the preparation of gold clusters consisting of <10 atoms. These samples have been imaged with atomic resolution by aberration-corrected scanning transmission electron microscopy. The images show that treatment of the supported mononuclear complexes at 318 K in flowing helium caused aggregation of the gold into clusters of 2-6 atoms, present with unconverted individual site-isolated mononuclear gold species and in the absence of any larger nanoparticles. Treatment of the sample at a higher temperature (373 K) in flowing helium resulted in the formation of gold clusters with diameters of 0.58 +/- 0.15 nm (containing roughly 10 Au atoms), again in the absence of larger nanoparticles. Upon exposure of the supported nanoclusters to the electron beam, they underwent aggregation to gold clusters approximately 1 nm in average diameter, as shown in consecutive STEM images.