Studies are reported of the kinetics of photosensitized reactions involving the transfer of absorbed light energy from humic substances to various trace organic chemicals in water. The photoisomerization of 1,3-pentadiene and the photooxygenation of 2,5-dimethylfuran were used to probe the nature and concentrations of the excited species that mediate humus-sensitized photoreactions of aquatic pollutants. Evidence is presented that the photosensitized reactions of pentadiene and dimethylfuran do not involve binding of the chemicals by the humic substances. Kinetic results indicate that the key steps in both photoreactions involve the transfer of electronic energy from triplet states of the humic substances. Up to half of these triplets are estimated to have energies of at least 250 kJ/mol, sufficiently high to transfer energy to polycyclic aromatic hydrocarbons, nitroaromatic compounds, conjugated dienes, and other types of chemicals. Steady-state concentrations of the humus-derived triplets are estimated to typically fall in the 10 to the -15th power -10 to the -13th power M range in the upper layers of sunlight-irradiated natural waters. The concentrations are proportional to UV absorption coefficients of the water. Action spectra for the photosensitized reactions indicate that solar ultraviolet radiation is most important in inducing the reactions. Results of the study are used to estimate maximum rates of reactions photosensitized by humic substances.