An isoenergetic molecular dynamics simulation is done to investigate two fundamental atomistic mechanisms responsible for rapid diffusion in nanoclusters. Using a semi-empirical potential we successfully reproduce the so-called Spontaneous Alloying and Spontaneous Mixing phenomena, which have been found in experiments for bimetallic and ternary alkali halide clusters, respectively. We found that a novel mechanism, named here the Surface Peeling Mechanism, is of primary importance for Spontaneous Alloying, while the Vacancy Mechanism is dominant in Spontaneous Mixing. We also show that the dominant mechanism responsible for rapid diffusion is related to the size dependence of vacancy formation energy. (C) 2011 Elsevier B.V. All rights reserved.