Interconversion of 2-deoxy-2-fluoro-D-glucose (FDG) and 2-deoxy-2-2-fluoro-D-mannose (FDM) catalyzed by acid has been detected by 19F-NMR method. Although FDG and FDM were stable towards 1 N hydrochloric acid, in stronger acidic media the two hexoses underwent epimerization at C-2.