The relative electrophilicity of different peroxo functions is deduced based on crystal structure correlation analysis and high-level ab initio theoretical calculations on a series of model peroxo compounds. Using electron density analysis, it is shown that peroxo functions in some organic compounds (especially dioxiranes) have a pronounced reversal of polarity resulting in a substantial electrophilic character, while in transition metal peroxides they only have an intermediate electrophilicity. The charge density of a transition metal oxidation catalyst, oxoperoxo(pyridine-2,6-dicarboxylato)(hexamethylphosphoramide)molybdenum(VI), 1, has been determined from combined analysis of 20 K X-ray and neutron diffraction data. The good comparison with results of theoretical calculations (at the HF and B3LYP levels of theory) validates the two approaches and testifies to the suitability of experimental methods even in the presence of heavy atoms. The analysis shows that the Mo-O-peroxo bond contains considerable covalent character as revealed by the short Mo-O distance, the large electron density along the bond path, and the negative energy density. In 1, the O-O distance, the atomic charges, and the electrostatic potential around the peroxo group are different from those of dioxiranes. During a direct interaction with olefins, a substantial repolarization of the group is expected to occur, possibly favored by weaker M-O bonds.