The formation of Ag,Se phase has been studied in the case of high-temperature successive deposition of polycrystalline silver thin films and selenium. It is the first time that the formation of very large Ag,Se single crystals of (001) orientation on polycrystalline Ag films are observed and discussed. Continuous 35 nm thick silver films prepared on amorphous substrates have been reacted with various amounts of selenium at different substrate temperatures above and below the bulk polymorphic phase transition temperature of Ag,Se (T-c = 133 degrees C). All structural investigations were carried out on films transformed to or formed directly in the low temperature orthorhombic phase. TEM investigations revealed that the continuous Ag,Se layers undergoing phase transformation during cooling to room temperature consisted of large (100-300 mu m) (001) oriented orthorhombic crystals of various azimuthal orientations. No preferred orientation was found in discontinuous Ag,Se films. Without phase transformation the orientation remained random with grain sizes below 1 mu m. Strong thickness fluctuations showing up in the form of island-channel-like morphology within the Ag,Se crystallites has been observed in all continuous layers. The formation of large (001) oriented single crystals is proposed to be related to the polymorphic phase transition from body-centred cubic to orthorhombic phase taking place during the cooling of the film after sample preparation,