Abstract Epitaxial fcc UO 2 films were obtained on (100) vacuum cleavage faces of NaCl, KCl, KBr, NaF and LiF, and a transmission electron microscope observation was made. Films of (100) orientations having extra spots due to micro-twins were most frequently observed for substrate temperatures of 350 to 550 °C for NaCl, 350 to 450 °C for KCl, 300 to 400 °C for KBr and 500 to 550 °C for LiF. Films prepared on NaF in the temperature range from 350 to 550 °C were observed to have a perfect (100) orientation without extra spots, while those prepared on LiF at 500 °C were sometimes observed to have overlapping (100) and (211) orientations. Registration of fission fragment tracks as straight lines about 100–200 A wide, of bright contrast were observed in films irradiated with thermal neutrons.