Gold nanoparticles have a high catalytic activity for CO oxidation at low temperatures providing they remain less than 5nm in diameter. Their structure and stability and the growth processes that occur during heating have been investigated using Angstrom resolution in-situ double aberration corrected transmission electron microscopy with a JEOL JEM-2200FS. Colloidal Au nanoparticles suspended in water, with mean diameters of 2nm and 5.6nm, have been deposited onto 3.5nm thin carbon supported on holey carbon grids and onto Si3N4 membranes. Dynamic in-situ high resolution AC-TEM images show competitive sintering processes on the different supports. Whilst the 5.6nm particles were observed to be very stable on the carbon, the 2nm particles showed sintering predominantly through particle migration and coalescence, with particle migration occurring as early as ∼200°C, peaking at ∼500°C. In contrast Au nanoparticles on Si3N 4 membranes were observed to coalesce at ∼180°C, before Ostwald Ripening became the dominant growth process at higher temperatures. It is believed that atoms and small clusters migrate away from their original particle before becoming trapped on the Si3N4 substrate. These trapped clusters then build up to form the small particles observed, before having sufficient energy to continue to migrate and join another larger particle at ∼500°C.