Crystalline ZnO quantum dots have been synthesized by hydrolysis of zinc acetate dihydrate with lithium hydroxide in ethanolic solution, By varying different parameters of the synthesis process, the size of the ZnO particles can be controlled. Detailed investigation of the ripening of the nanoparticles evidenced that despite of the well-known influence of ageing temperature and time, the presence of the reaction byproduct lithium acetate strongly affects the ripening behaviour. In particular, the particle size can be almost completely arrested by the removal of this byproduct via reversible flocculation of the ZnO nanoparticles using heptane as an antisolvent. A closer analysis of the repeated washing process shows an initial improvement of the colloidal stability of the ZnO nanoparticles during the first purification cycle as it mainly removes the lithium acetate from the suspension and not the stabilizing acetate groups directly bound to the particle surface. With further washing the remaining acetate ligands are unable to maintain the stabilization against agglomeration of the ZnO nanoparticles. Thus, there exists an optimum between purification progress and colloidal stability. These findings are also confirmed by calculations according to the DLVO theory, which show that there exists nearly no primary minimum of small ZnO nanoparticles below 5 nm in the presence of stabilizing acetate ions whereas the decrease in acetate ions bound to the particle Surface leads to a more and more pronounced primary minimum. The present work is of particular significance for the preparation of purified colloidal ZnO nanoparticles for studies of their electrical and optical properties with respect to their wide range of potential applications. (C) 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.