We report oil the first in situ scanning tunneling microscopy (STM) measurements showing Ostwald ripening of InAs quantum dots (QDs) grown on Si-doped (001) GaAs by metal-organic vapor phase epitaxy (MOVPE). During an annealing step in the reactor under arsenic overpressure immediately after QD growth at (475 degrees C) the change in QD density and size distribution could be observed directly in a sequence of in Situ STM images over 50 min. The density of the InAs QDs decreases reciprocally during the annealing step, which agrees well with Ostwald ripening limited by indium attachment/ detachment. (C) 2008 Elsevier B.V. All rights reserved.