A segmented flow automated method with on-line photo-oxidation for the determination of dissolved organic phosphorus + soluble reactive phosphorus (DOP + SRP) in seawater and fresh water is described here. A low-power lamp was used for a compact, easy-to-handle and low-ozone-producing manifold. The influence of seawater matrix components was studied in detail using natural seawater and salt solutions spiked with DOP model compounds. Bromide was found to be the most inhibitory species in seawater. The work shows that most salt solutions referred to as artificial seawater are not satisfactory model matrices to test the actual seawater matrix effect. Since DOP recovered in undiluted seawater samples was about half that obtained in fresh water samples, the described method includes a 5- to 6-fold dilution of seawater samples. This simple procedure overcomes matrix effects and provides satisfactory DOP recovery. No pH effect was found in the 6-9 range corresponding to most natural waters. The standard deviation was 0.007 mu mol l(-1) and the limit of detection 0.02 mu mol l(-1). Linearity was tested up to 5 mu mol l(-1) of DOP, i.e. far above naturally occurring values. A throughput of 20 samples per hour is easily achieved. (C) 2001 Elsevier Science B.V. All rights reserved.