Grazing incidence x-ray irradiation of a Langmuir monolayer deposited at the interface between an aqueous silver salt solution and the air leads to the formation of a metallic silver layer of thickness about 4.5 nm, in agreement to the x-ray penetration depth at the air - water interface. Moreover using the scattering and fluorescence induced by the x-ray, we show that it is possible to follow the kinetics of formation of the layer. It appear that the silver layer is formed in two steps: first, the formation of small oriented cristallites, and second, the coalescence of the cristallites forming large plates.