[en] Electron diffraction work has been carried out on yttrium films prepared in a vacuum of 5 x 10^-7Torr during evaporation. The crystal structure of thin yttrium films (300 A thick or less) is mainly face-centred-cubic with asub(O) = 5.25 +- 0.05 A which can be attributed to cubic yttrium dihydride. The amount of the FCC phase decreases with increasing film thickness and films about 600 A thick are mainly HCP α yttrium. When air and hydrogen are admitted to the system the films react to form oxide and hydride respectively. The time dependence of the electrical resistance of these films has been studied and it is found that in the case of hydrogen contamination there is a minimum in the resistance-time curve. Electron diffraction observations have also been made. All the films transform to Y₂0₃ when heated in an intense electron beam inside the electron microscope. The resistivity results are discussed in terms of the known electrical resistivities of compounds of yttrium. (author)