The very early stage of nano-BaTiO3 sintering was monitored using a number of experimental methods. It was found that in situ impedance spectroscopy provides the most sensitive probe to determine the beginning of the sintering process. Dramatic changes in the impedance can be detected in situ on time scales of minutes at temperature as low as 500 degrees C. The mechanisms behind these changes hardly influence the surface area and do not result in shrinkage. A microscopic model of the process that complies with the experimental observations is suggested and discussed. This includes oxygen vacancy formation in the very early low temperature stage, and particle rearrangement at the beginning of the shrinkage stage. (c) 2006 The Electrochemical Society.