The exchange kinetics between monovalent and multivalent counterions under a charged Langmuir monolayer of eicosylamine has been monitored by X-ray reflectivity over a period of 24 h, at 5 min time intervals and with a spatial resolution of 5 Angstrom. This time resolution is quite remarkable considering that the ionic layer contains < 10(14) ions per cm(3) and is only 10 Angstrom thick. The clue was to use two ionic species with a very large electron density difference. Chloride was selected as the singly charged anion whereas the multiply charged anion was phosphotungstate, a large, electron-dense heteropolyanion of the Keggin salts family. Our data, in combination with surface pressure measurements and Brewster angle microscopy, show that the counterion replacement takes place in three consecutive stages. There is a long initial incubation period of 45 min, during which there is no substantial ion exchange. All replacement takes place later and over a brief time interval of approximately 30 min. Finally, there is a reorganization stage during which the ionic layer rearranges itself to form a well-ordered, dense, and monomolecular-thick. layer.