Solid-state reaction processes and atomic ordering in Cu/Au bilayer nanofilms (with the atomic ratio Cu:Au»50:50) have been studied in situ by the methods of transmission electron microscopy and electron diffraction in the process of heating from room temperature up to 700ºС at a heating rate of 4-8ºС/min. The solid-state reaction between the nanolayers of copper and gold has been established to begin at 180ºС. The process of atomic ordering has been shown to start simultaneously with the process of the formation of the disordered phase of Cu₅₀Au₅₀ at 245ºС. The formation processes of the ordered phases of: CuAuI (L1₀ superstructure) and CuAuII (long period superstructure) have been studied, as well as the phase transition processes: disorder - order (the transition of the disordered structure into the ordered one) and order - disorder (the transition of the ordered structure into the disordered one).