The catalytic activity of onium compounds was studied in the homogeneous liquid phase oxidation of cumene and α-pinene from the viewpoint of the effect of the structure of the oniums on the activity. Among a number of onium compounds, sulfonium and phosphonium possessing the lowest vacant d-orbitals were found most effective. Among the substituents (attached to the central atom) and the counter anions, the electron donor group such as methyl, ethyl or larger alkyl and phosphate or sulfate anion made the activity greater. The roles of the substituents and the anion may be understood from an electron transfer to the onium positive center in which the vacant orbitals become partially occupied. Finally, the interaction between the catalyst and molecular oxygen was discussed, concluding that the catalytic activity might be attributed to the electron migration between them.