Quantification of photochemical O2 uptake provides a measure of total chemical photooxidation of dissolved organic matter (DOM). Here we study this process and present estimates suggesting that photooxidation has the potential to significantly modify marine DOM pools, complementing or exceeding oxidation via coupled chemical‐biological pathways. We measured apparent quantum yields (AQYs) of photobleaching, O2 uptake, and H2O2 production in several coastal marine samples and in dilutions of a tropical estuarine water with oligotrophic seawater. O2‐loss AQYs varied little among samples or with dilution but decreased linearly from 1.2×10–3 at 300 nm to 0.3×10–3 at 400 nm and dropped about threefold to near‐constant values with increasing absorbed light dose. H2O2 production, about 45% of O2 uptake, showed similar dependencies, whereas singlet oxygen (O2 (1Δg)) reactions contributed less than 1% of O2 uptake for typical coastal water. Implications of these findings for photochemical O2, H2O2, and DOM cycling are discussed.Modeling the dose‐dependence of O2 loss and photobleaching at 310 nm required three DOM pools. In the simplest case, about 90% is a weakly absorbing, low‐AQY pool of DOM admixed with two similar‐sized pools of more photochemically reactive DOM. This result suggests that rigorously extrapolating laboratory data to the environment requires detailed mapping of dose‐wavelength‐photobleaching AQY surfaces.Action spectra and DOM flux estimates for coastal photooxidative chemistry were derived. Site‐specific potential rates are comparable to available in situ data. Globally, the DOM photolysis capacity appears to be larger than estimated coastal DOM inputs, especially in tropical and temperate areas, including areas with maximal DOM inputs.