AbstractThe mechanism of formation of initial condensate droplets for dropwise condensation in the nanometer scale is still undetermined. In this paper, magnesium was used as the condensation surface since it can react with condensate to produce magnesium hydroxide, which can be regarded as the marker of the initial condensate state on the surface. Then an electron probe microanalyzer (EPMA) and scanning electron microscope (SEM) were used to scan the variation of the chemical compositions on the surfaces in nanometer scale before and after the initial condensation. The results showed that the oxygen contents on the test surfaces increased obviously with subcooling and condensation time after the initial dropwise condensation. In order to further analyze the variation of oxygen content occurring on the whole surface or only at the partial locations, the reaction dynamic relationship of magnesium and the condensate was found. And the calculated result indicated that the area ratio occupied by condensate was less than 1. Meanwhile the detected results of EMPA showed that oxygen on the test surface was distributed non‐uniformly. As a result, the formation mechanism of the initial condensate drops for dropwise condensation agrees with the hypothesis of nucleation sites in nanometer scale. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20377