Humic substances (HS) acting as photosensitizers can generate a variety of reactive species, such as OH radicals and excited triplet states ((HS)-H-3*), promoting the degradation of organic compounds. Here, we apply compound-specific stable isotope analysis (CSIA) to characterize photosensitized mechanisms employing fuel oxygenates, such as methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE), as probes. In oxygenated aqueous media, Delta (Delta delta H-2/Delta delta C-13) values of 23 +/- 3 and 21 +/- 3 for ETBE obtained by photosensitization by Pahokee Peat Humic Acid (PPHA) and Suwannee River Fulvic Acid (SRFA), respectively, were in the range typical for H-abstraction by OH radicals generated by photolysis of H2O2 (Delta = 24 +/- 2). However, (3)HS* may become a predominant reactive species upon the quenching of OH radicals (Delta = 14 +/- 1), and this process can also play a key role in the degradation of ETBE by PPHA photosensitization in deoxygenated media (Delta = 11 +/- 1). This is in agreement with a model photosensitization by rose bengal (RB2-) in deoxygenated aqueous solutions resulting in one-electron oxidation of ETBE (Delta = 14 +/- 1). Our results demonstrate that the use of CSIA could open new avenues for the assessment of photosensitization pathways.