Photochemical production of hydroxyl radical (·OH) from algal organic matter (AOM) collected from Lake Torrens in South Australia was examined using a sunlight simulator. The two AOM isolates featured lower molecular weight, lower chromophoric content, and lower SUVA254 (0.7 and 0.9, L mgC-1 m-1) than the reference Suwannee River hydrophobic acid (SR-HPO), they had considerably higher apparent quantum yields (ϕNOMOH, 3.03 × 10-5 and 2.18 × 10-5) than SR-HPO (0.84 × 10-5). Fluorescence excitation-emission matrix (FEEM) showed that the major components in the AOM were aromatic protein-like and soluble microbial substances. Unique formulas of the two AOM isolates as compared to SR-HPO were revealed using FTICR-MS and classified into four areas, namely protein-like molecules with low O/C (H/C > 1.5, O/C: 0.2-0.4), lignin-derived moieties with low O/C (H/C:1.0-1.5, O/C: 0.1-0.3), protein-like molecules with high O/C (H/C > 1.5, O/C: 0.5-0.7), and carbohydrate derivatives (H/C > 1.5, O/C > 0.7). These unique AOM moieties characterised utilizing FEEM and FTICR-MS were tentatively postulated to contribute to the high ϕNOMOH. To the best of our knowledge, this is the first study performed to both evaluate natural AOM as an efficient photosensitiser of ·OH and propose AOM moieties responsible for the high ϕNOMOH.