The present invention relates to a method for improving stability of non fluoridated sugar derivatives, and in particular glucose derivatives such as 1,3,4,6-tetra-O-acetyl-2-O-trifluoromethanesulfonyl-.beta.-D-mannopyra- nose which are used as precursors for production of radiofluoridated sugar derivatives for use in in vivo imaging procedures such as positron emission tomography (PET). The method comprises storing the non fluoridated sugar derivative in an organic solvent. The resultant formulations of the non fluoridated sugar derivative and cassettes for automated synthesis apparatus comprising the same are also claimed.