The very early stages of Pb deposition on Si(111)7x7 surfaces have been studied by means of scanning tunneling microscopy. Variable temperature measurements combined with unusually long periods of imaging time have made it possible to analyze the dynamics of Pb atom pairs. The observations show that the pairs are actually mobile at room temperature within the half (7x7) unit cells where they are adsorbed. An activation energy of 0.40+/-0.04 eV has been measured for this thermally activated process.