The Schlieren or refractive index (RI) effect is a major problem in the determination of dissolved reactive phosphorus in estuarine waters using conventional flow injection (FI) manifolds with sample injection. This is because differences in RI between the injected sample zone and the carrier stream give rise to a lensing effect which is superimposed on the blank response and causes significant error in quantitation. A simple reversed flow injection (rFI) manifold using spectrophotometric detection which removes these quantitation errors is reported. Acidic molybdate is injected into a sulfuric acid carrier stream of the same refractive index and sequentially merged with sample and reductant (ascorbic acid). Reduction of phosphomolybdate to phosphomolybdenum blue is carried out in a coil thermostated at 60 °C. Dissolved reactive phosphorus has been successfully determined in estuarine waters with salinities ranging from 0 to 30‰ using calibration standards prepared in deionized water, with a detection limit of 2 µg l^–1 PO₄–P and a linear range of 2–100 µg l^–1 PO₄–P (r² = 0.9998).