The separate determination of the rate constants of the competitive formation of O-2((1)Sigma (+)(g)), O-2((1)Delta (g)), and O-2((3)Sigma (-)(g)) in the deactivation of the triplet state by O-2 Of sensitizers with incomplete intersystem crossing is exemplarily demonstrated with naphthalene. The method is based on triplet-triplet energy transfer in CCl4 from a primary sensitizer to the sensitizer of interest. Because of the very fast quenching of O-2((1)Sigma (+)(g)) by hydrocarbons, the conditions of these experiments are rather stringent. All the different energy transfer steps involved have to be quantitatively determined.;This is done by examining the phosphorescence of the primary sensitizer as well as the (1)Sigma (+)(g) --> (3)Sigma (-)(g), (1)Sigma (+)(g) --> (1)Delta (g), and (1)Delta (g) --> (3)Sigma (-)(g) emissions of O-2 in air-saturated CCl4.