Radiolysis is the process by which radioactively labeled compounds degrade. Many positron emission tomography (PET) radiopharmaceuticals produced with high radioactive concentrations and specific activities exhibit low radiochemical purity because of radiolysis. Little data exist that describe the radiolytic decomposition of 2-[(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG). The objective of our study was to profile the degradation of [(18)F]FDG at various radioactive concentrations by measuring radiochemical purity at different time intervals and to study the effects of ethanol, a well-known reductant stabilizer of [(18)F]FDG preparations.