Oriented aggregation is an important, nonclassical crystal growth mechanism by which nanocrystals grow, defects are formed, and unique-often symmetry-defying-crystal morphologies can be produced. This growth mechanism involves the irreversible and crystallographically specific self-assembly of primary nanocrystals and results in the formation of new single crystals, twins, and intergrowths. This paper presents data showing that growth by oriented aggregation is consistent with second-order kinetics with respect to the concentration of the primary nanoparticles and demonstrates that the overall rate constant for growth by oriented aggregation increases dramatically with decreasing primary particle size. Results are discussed in the context of DLVO interactions. The experimental results explain the common observation that. growth by oriented aggregation slows as a function of continued crystal growth. (C) 2007 Elsevier B.V. All rights reserved.