Nucleation, growth and coalescence of 3D Au-Ge droplets on Ge(1 1 1) surface have been studied in real time by low energy electron microscopy. The surface diffusion energy barrier of Au on Ge(1 1 1) is estimated as eV from the dependence of the number of droplets per surface unit with temperature and Au deposition rate. At long time scale it is also shown that the droplets have a diffusive motion that results in dynamic coalescence events. From the decrease of the droplet number, the size-dependence of the diffusion coefficient of the droplets is shown to decay as where R is the droplet radius. This result indicates that the spontaneous motion of the droplets is controlled by an interfacial mass-transport phenomenon.