Stable scanning tunneling microscopy (STM) images during the early stages of air exposure of the passive film on iron were successfully captured by a humidity- controlled method. The passive film formed at -0.4 V SCE in pH 8.4 borate buffer solution was first kept in humid air (47% relative humidity) at 15 degrees C for 24 h and then exposed to dry air (< 30% RH) at room temperature. Upon exposure to the dry air, the surface structure underwent a distinctly three- stage transformation, concluding with a grain ripening process that has not previously been observed in passive films. (c) 2005 The Electrochemical Society.