The observation of anion order is indispensable for the investigation of oxyfluorides. However, the negligible contrast between O2- and F- in both X-ray and neutron diffraction obscures the distinct anion sites for Rietveld refinement. Therefore, the difference in the chemical bonding of M-O2- and M-F- is the key to determining anion order. In this study, bond-valence-sum calculations and determination of the electron density distribution by the maximum entropy method illustrated anion order in the newly synthesized oxyfluoride Pb2Ti4O9F2. These results demonstrate a promising method to determine anion order in mixed anion systems.