An on-line flow-injection system for the determination of total phosphorus in waters and wastewaters is described. Digestion was performed using a UV photoreactor and a thermal digestion unit connected in series. The orthophosphate formed was detected as phosphomolybdenum blue after on-line filtration. Reagent and digestion conditions required to give optimal oxidation and hydrolysis of organic and condensed model compounds are reported. The flow-injection method gave complete recovery for all the model organic P compounds tested, and >85% recovery for condensed phosphates. When the proposed method was applied to a series of wastewater samples, the results obtained were in close agreement with those from a conventional batch digestion method. The technique was rapid (8 samples h−1 4 replicates), with a linear range of interest for wastewaters 0–18 mgPl−1 and a detection limit of 0.15 mgPl−1.