Phenolic moieties in dissolved organic matter (DOM) play important roles as antioxidants in oxidation processes in natural and engineered systems. This work presents an automated and highly sensitive flow injection analysis (FIA) system coupled to both spectrophotometric and electrochemical detection to quantify electron-donating phenolic moieties in DOM by determining the number of electrons that these moieties transfer to an added chemical oxidant, the radical cation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS(•+)). The FIA system was successfully validated using Trolox as a redox standard. Highest method sensitivity was attained when combining the FIA with chronoamperometric detection, resulting in limits of quantification of picomolar amounts of Trolox and nanogram amounts of DOM (corresponding to solutions with <1 mg carbon per liter). The analysis of DOM isolates showed a strong linear correlation between the number of electrons donated and their titrated phenol contents, supporting oxidation of phenols by ABTS(•+). The broad application spectrum of the FIA system to dilute natural DOM samples was illustrated by analyzing water samples collected from northern peatlands and by monitoring the oxidation of phenols in one peat sample upon incubation with a phenol oxidase. The superior analytical capability of the FIA system allows quantifying phenols and monitoring phenol dynamics in dilute DOM samples.