The oxygen quenching rate constants of aliphatic ketones (acetone, 2-pentanone, 2-hexanone, 4-heptanone) and diketones (biacetyl and camphorquinone) have been determined for the singlet- and triplet-excited state by time-resolved fluorescence and triplet transient absorption spectroscopy, and the quantum yields for singlet oxygen formation were measured. It was observed that the quenching rate constants for the singlet state of aliphatic ketones are considerably smaller than for their triplet states. The efficiencies for singlet oxygen production of the aliphatic ketones were 0.2-0.3, while those of most diketones fall in the 0.4-0.6 range. The changes in the efficiencies for singlet oxygen production and of the oxygen-quenching rate constants for singlet- and triplet-excited n,pi* states, ketones and azoalkanes are discussed in terms of variations in excited state energies and charge-transfer interactions with oxygen.