This article contains experimental reflection and transmission secondary emission yield curves obtained from a 475 Å unbacked MgO film. The film has a transmission yield maximum of 2.6 at a primary energy of 3 kev. An analysis, employing experimental transmission retarding potential characteristics also obtained in this study, proves a lack of proportionality between the production of secondaries and the energy dissipated by the primary beam in MgO. The extreme range of a 3600‐ev primary in MgO is found to be 475 Å.